Function plan 

pub fn plan(
    main_doc: &LemmaDoc,
    all_docs: &[LemmaDoc],
    sources: HashMap<String, String>,
) -> Result<ExecutionPlan, Vec<LemmaError>>

Builds an execution plan from Lemma documents.

The sources parameter maps source IDs (filenames) to their source code, needed for extracting original expression text in proofs.