code: def p_length(self, p):
          'length : LBRACKET expression COLON expression RBRACKET'
          p[0] = Length(p[2], p[4], lineno=p.lineno(1))
          p.set_lineno(0, p.lineno(1)f p_length(self, p):
          'length : LBRACKET expression COLON expression RBRACKET'
          p[0] = Length(p[2], p[4], lineno=p.lineno(1))
          p.set_lineno(0, p.lineno(1))
docs: length : LBRACKET expression COLON expression RBRACKET