type Place struct {
		Code string
	}
	type Segment struct {
		Origin Place
	}
	type Helpers struct {
		PlaceEq func(p Place, s string) bool
	}
	type Request struct {
		Segments []*Segment
		Helpers
	}

	code := `Segments[0].Origin == "MOW" && PlaceEq(Segments[0].Origin, "MOW")`

	program, err := expr.Compile(code, expr.Env(&Request{}), expr.Operator("==", "PlaceEq"))
	if err != nil {
		fmt.Printf("%v", err)
		return
	}