code: func NumberOperand(x ast.Value, pos int) (ast.Number, error) {
  	n, ok := x.(ast.Number)
  	if !ok {
  		return ast.Number(""), NewOperandTypeErr(pos, x, "number")
  	}
  	return n, nil
  }
docs: // NumberOperand converts x to a number. If the cast fails, a descriptive error is
  // returned.