Rename the constructor for function definitions from `define` to `fdef` (short for function definition)
Rename the type `Def` to `FDef`
Correct the misuse of the abbreviation `Sig` and `sig` (for Signature and signature) by renaming them to `FDefs` and `fdefs` respectively
Replace less disinformative and misinformative names
Replace variable declarations in blocks with variable definitions