=====================================
Simple Function Definition
=====================================
function simple () {
    return 1;
}
---
 (source_file
      (function_definition
        (identifier)
        (parameter_list)
        (function_body
          (return_statement
            (int_literal)))))
=====================================
Simple Function with Arguments
=====================================
function add (a, b) {
    return a + b;
}
---
 (source_file
      (function_definition
        (identifier)
        (parameter_list
          (parameter
            (identifier))
          (parameter
            (identifier)))
        (function_body
          (return_statement
            (binary_expression
              (identifier)
              (identifier))))))