parameters: (p)
docstring: expression : expression '+' expression
                    | expression '-' expression
                    | expression '*' expression
                    | expression '/' expression
docstring_summary: expression : expression '+' expression
                    | expression '-' expression
                    | expression '*' expression
                    | expression '/' expression
docstring_tokens: expression : expression + expression | expression - expression | expression * expression | expression / expression
function: def p_expression_binop(p):
      '''expression : expression '+' expression
                    | expression '-' expression
                    | expression '*' expression
                    | expression '/' expression'''
      if p[2] == '+'  : p[0] = p[1] + p[3]
      elif p[2] == '-': p[0] = p[1] - p[3]
      elif p[2] == '*': p[0] = p[1] * p[3]
      elif p[2] == '/': p[0] = p[1] / p[3]
function_tokens: def p_expression_binop ( p ) : if p [ 2 ] == '+' : p [ 0 ] = p [ 1 ] + p [ 3 ] elif p [ 2 ] == '-' : p [ 0 ] = p [ 1 ] - p [ 3 ] elif p [ 2 ] == '*' : p [ 0 ] = p [ 1 ] * p [ 3 ] elif p [ 2 ] == '/' : p [ 0 ] = p [ 1 ] / p [ 3 ]