diff --git a/tools/src/bin/parser/main.rs b/tools/src/bin/parser/main.rs
index b37e06d..552cbe2 100644
--- a/tools/src/bin/parser/main.rs
+++ b/tools/src/bin/parser/main.rs
@@ -1,21 +1,25 @@
 use clap::Parser;
 
-use tools::Repo;
-
 #[derive(Debug, clap::Parser)]
 #[command(version, about, long_about = None)]
 struct Args {
     /// The name of the recipe to build.
     target: String,
 
-    /// Directory containing the repo of all repositories.
+    /// Directories containing repos.
     #[arg(short, long)]
-    repo: String,
+    // repo: String,
+    repo: Vec<String>,
 }
 
 fn main() -> anyhow::Result<()> {
     let args = Args::parse();
-    let repo = Repo::new(&args.repo).unwrap();
-    repo.compile(&args.target)?;
+    // let repo = tools::Repo::new(&args.repo).unwrap();
+    // repo.compile(&args.target)?;
+    let mut repos = tools::Repos::default();
+    for repo_path in &args.repo {
+        repos.add_repo(repo_path)?;
+    }
+    repos.compile(&args.target)?;
     Ok(())
 }
diff --git a/tools/src/lib.rs b/tools/src/lib.rs
index 09890d9..c9b6ed3 100644
--- a/tools/src/lib.rs
+++ b/tools/src/lib.rs
@@ -5,6 +5,9 @@ pub mod repo;
 pub use repo::Repo;
 pub use repo::RepoLoadError;
 
+pub mod repos;
+pub use repos::Repos;
+
 pub mod recipe_id;
 pub use recipe_id::RecipeId;
 pub use recipe_id::RecipeIdParseError;
diff --git a/tools/src/repos.rs b/tools/src/repos.rs
new file mode 100644
index 0000000..753712c
--- /dev/null
+++ b/tools/src/repos.rs
@@ -0,0 +1,268 @@
+use std::io::Write;
+use std::process::Command;
+use std::str::FromStr;
+
+use thiserror::Error;
+
+use crate::recipe::Recipe;
+
+use crate::recipe_id::RecipeId;
+use crate::recipe_id::RecipeIdParseError;
+
+use crate::repo::Repo;
+
+#[derive(Default)]
+pub struct Repos {
+    repos: std::collections::HashMap<String, Repo>,
+}
+
+#[derive(Debug, Error, PartialEq)]
+pub enum RecipeLookupError {
+    #[error(transparent)]
+    RecipeIdParseError(RecipeIdParseError),
+    #[error("recipe not found: {0}")]
+    RecipeNotFound(String),
+    #[error("repo not found: {0}")]
+    RepoNotFound(String),
+}
+
+impl From<RecipeIdParseError> for RecipeLookupError {
+    fn from(val: RecipeIdParseError) -> Self {
+        RecipeLookupError::RecipeIdParseError(val)
+    }
+}
+
+#[derive(Debug, Error)]
+pub enum AddRepoError {
+    #[error("repo load error: {0}")]
+    RepoLoadError(#[from] crate::repo::RepoLoadError),
+    #[error("invalid repo name: {0}")]
+    InvalidRepoName(String),
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum RecipeCompileError {
+    #[error(transparent)]
+    RecipeLookupError(#[from] RecipeLookupError),
+    #[error(transparent)]
+    StdIoError(#[from] std::io::Error),
+    #[error("vitamin has no capital input: {0}")]
+    VitaminLacksCapital(String),
+    #[error("vitamin capital has wrong unit: {0}")]
+    VitaminCapitalHasWrongUnit(String),
+    #[error("input produces no output: {0}")]
+    InputProducesNoOutput(String),
+}
+
+impl Repos {
+    pub fn add_repo(&mut self, repo_path: &str) -> Result<(), AddRepoError> {
+        let repo_pathbuf = std::path::PathBuf::from(repo_path);
+        match repo_pathbuf.file_name() {
+            Some(repo_name) => {
+                let repo = Repo::new(repo_path)?;
+                let repo_name = repo_name
+                    .to_str()
+                    .ok_or_else(|| AddRepoError::InvalidRepoName(repo_path.into()))?;
+                self.repos.insert(repo_name.into(), repo);
+                println!("added repo {repo_name}");
+                Ok(())
+            }
+            None => {
+                todo!();
+            }
+        }
+    }
+
+    // Recipe names are of the form "[repo:]recipe".
+    // If repo is specified, look only in that repo.
+    // If repo is not specified, search all repos.
+    pub fn get_recipe(&self, recipe_name: &str) -> Result<&Recipe, RecipeLookupError> {
+        let recipe_id = RecipeId::from_str(recipe_name)?;
+        match recipe_id.repo {
+            Some(repo) => {
+                let repo = match self.repos.get(&repo) {
+                    Some(repo) => repo,
+                    None => return Err(RecipeLookupError::RepoNotFound(repo)),
+                };
+                repo.get_recipe(&recipe_id.recipe)
+                    .ok_or_else(|| RecipeLookupError::RecipeNotFound(recipe_name.into()))
+            }
+            None => {
+                for (_repo_name, repo) in self.repos.iter() {
+                    if let Some(recipe) = repo.get_recipe(recipe_name) {
+                        return Ok(recipe);
+                    }
+                }
+                Err(RecipeLookupError::RecipeNotFound(recipe_name.into()))
+            }
+        }
+    }
+
+    pub fn compile(&self, target: &str) -> Result<(), RecipeCompileError> {
+        let recipe = self.get_recipe(target)?;
+        let puml_filename = format!("{target}.puml");
+        let mut puml_file = std::fs::File::create(&puml_filename)?;
+        writeln!(puml_file, "@startuml")?;
+        writeln!(puml_file, "object {}", target)?;
+        self.compile_inner(&mut puml_file, target, recipe, 4)?;
+        writeln!(puml_file, "@enduml")?;
+        let output = Command::new("plantuml")
+            .arg("-v")
+            .arg(&puml_filename)
+            .output()?;
+        if !output.status.success() {
+            println!("{:#?}", output);
+        }
+        Ok(())
+    }
+}
+
+impl Repos {
+    fn compile_inner(
+        &self,
+        puml_file: &mut std::fs::File,
+        recipe_name: &str, // the name of the thing we're making
+        recipe: &Recipe,   // the recipe for the thing we're making
+        indent: usize,
+    ) -> Result<(), RecipeCompileError> {
+        for (input_name, input_info) in recipe.inputs.iter() {
+            let input_recipe = self.get_recipe(input_name)?;
+
+            // A Recipe whose only input is Capital is a Vitamin.
+            let cost = match input_recipe.is_vitamin() {
+                true => {
+                    let amount_needed = input_info.quantity;
+
+                    // FIXME: for now Vitamins must have exactly one Input, and it must be Capital.
+                    assert_eq!(input_recipe.inputs.len(), 1);
+                    let input_capital = input_recipe.inputs.get("capital").ok_or_else(|| {
+                        RecipeCompileError::VitaminLacksCapital(input_name.into())
+                    })?;
+                    if input_capital.quantity.unit != Some(crate::recipe::Unit::USDollar) {
+                        return Err(RecipeCompileError::VitaminCapitalHasWrongUnit(
+                            input_name.into(),
+                        ));
+                    }
+                    let cost = input_capital.quantity.amount;
+
+                    let outputs = match &input_recipe.outputs {
+                        None => {
+                            return Err(RecipeCompileError::InputProducesNoOutput(
+                                input_name.into(),
+                            ));
+                        }
+                        Some(outputs) => outputs,
+                    };
+
+                    // FIXME: For now Vitamins must produce exactly one output.
+                    assert_eq!(outputs.len(), 1);
+
+                    let (_output_name, output_info) = outputs.iter().next().unwrap();
+                    let output_quantity = output_info.quantity;
+
+                    // compute the "unit cost" of this input
+                    let cost_each = cost / output_quantity.amount;
+                    let total_cost = amount_needed.amount * cost_each;
+
+                    // FIXME: Try to coerce the units here - if we buy
+                    // hose in rolls measured in feet, but we use lengths
+                    // of hose measured in meters, that should be ok.
+                    if let Some(output_unit) = output_info.quantity.unit {
+                        Some(format!(
+                            "${:.6} / {:?}\ntotal: ${:.6}",
+                            cost_each, output_unit, total_cost
+                        ))
+                    } else {
+                        Some(format!("${:.6} each\ntotal: ${:.6}", cost_each, total_cost))
+                    }
+                }
+                false => None,
+            };
+
+            let (input_object_name, input_object_declaration) = match input_recipe.is_vitamin() {
+                true => {
+                    // "Leaf" nodes get unique boxes.
+                    let uuid = uuid::Uuid::new_v4();
+                    let input_object_name = format!("uuid_{}", uuid.simple());
+                    (
+                        input_object_name.clone(),
+                        format!("\"{}\" as {}", input_name, input_object_name),
+                    )
+                }
+                false => (input_name.clone(), input_name.clone()),
+            };
+
+            writeln!(puml_file, "object {}", input_object_declaration)?;
+            match &input_recipe.action {
+                crate::recipe::Action::process(process) => {
+                    writeln!(puml_file, "{} : {:?}", input_object_name, process)?;
+                }
+                crate::recipe::Action::print => {
+                    writeln!(puml_file, "{} : print", input_object_name)?;
+                }
+                crate::recipe::Action::purchase(_purchase) => {
+                    writeln!(puml_file, "{} : buy", input_object_name)?;
+                }
+            }
+
+            if let Some(cost_str) = cost {
+                writeln!(puml_file, "{} : {:?}", input_object_name, cost_str)?;
+            }
+
+            write!(puml_file, "{} <|-- {}", recipe_name, input_object_name)?;
+            if input_info.quantity.unit.is_some() || input_info.quantity.amount != 1.0 {
+                write!(puml_file, " : quantity={:?}", input_info.quantity)?;
+            }
+            writeln!(puml_file)?;
+
+            if !input_recipe.is_vitamin() {
+                self.compile_inner(puml_file, input_name, input_recipe, indent + 4)?;
+            }
+        }
+
+        Ok(())
+    }
+}
+
+mod test {
+    #[test]
+    fn empty_repos() {
+        let empty_repos = crate::repos::Repos::default();
+        assert_eq!(
+            empty_repos.get_recipe("nonexistent".into()),
+            Err(crate::repos::RecipeLookupError::RecipeNotFound(
+                "nonexistent".into()
+            ))
+        );
+    }
+
+    #[test]
+    fn load_two_repos() -> Result<(), crate::repos::AddRepoError> {
+        let mut repos = crate::repos::Repos::default();
+        repos.add_repo("../modular-recipes/repos/fasteners")?;
+        repos.add_repo("../modular-recipes/recipes/peristaltic-pump")?;
+        assert_eq!(repos.repos.iter().count(), 2);
+        println!("{:?}", repos.repos.keys());
+        let _fasteners = repos.repos.get("fasteners").unwrap();
+        let _peristaltic_pump = repos.repos.get("peristaltic-pump").unwrap();
+        Ok(())
+    }
+
+    #[test]
+    fn get_recipe() {
+        let mut repos = crate::repos::Repos::default();
+        repos
+            .add_repo("../modular-recipes/repos/fasteners")
+            .unwrap();
+        repos
+            .add_repo("../modular-recipes/recipes/peristaltic-pump")
+            .unwrap();
+        repos
+            .add_repo("../modular-recipes/repos/minimal_hydroponics_setup")
+            .unwrap();
+        let _r = repos.get_recipe("peristaltic_pump").unwrap();
+        let _r = repos.get_recipe("m4_nuts").unwrap();
+        let _r = repos.get_recipe("fasteners:m4_nuts").unwrap();
+        let _r = repos.get_recipe("barb_fitting_1_2_inch").unwrap();
+    }
+}