type In struct {
	JPathExpr *JSONPath `parser:"@@"`
	ListExpr  *ListExpr `parser:"| @@"`
}

// Grammar for Operand:
//
// operand → multOp ( ("-" | "+") multOp )*
// multOp  → unary ( ("/" | "*" | "%") unary )*
// unary   → "-" unary | primary
// primary → Value | Variable | "(" expression ")"
//

// An Operand is a single term followed by an optional sequence of
// terms separated by +/-
type Operand struct {