CannotDefineFunctionInBodyError = object of ValueError

proc evaluate(ast: AST): int {....raises: [KeyError,
                                        CannotDefineFunctionInBodyError],
                               tags: [].}

proc evaluate(ast: AST; variables: var Table[string, int];
              functions: var Table[string, AST]): int {.
    ...raises: [KeyError, CannotDefineFunctionInBodyError], tags: [].}