func (m *hashmap) lookup(k hashable) Ivalue {
	if m != nil {
		hash := k.hash(m.keyType)
		for e := m.table[hash]; e != nil; e = e.next {
			if k.eq(m.keyType, e.key) {
				return e.Ivalue
			}
		}
	}
	return nil
}
lookup returns the Ivalue associated with key k, if present, or
Ivalue(nil) otherwise.
package interp
import (
	"go/types"
)
type hashable interface {
	hash(t types.Type) int
	eq(t types.Type, x interface{}) bool
}
type entry struct {
	key    hashable
	Ivalue Ivalue
	next   *entry
}
type hashmap struct {
	keyType types.Type
	table   map[int]*entry
	length  int  
}
func makeMap(kt types.Type, reserve int) Ivalue {
	if usesBuiltinMap(kt) {
		return make(map[Ivalue]Ivalue, reserve)
	}
	return &hashmap{keyType: kt, table: make(map[int]*entry, reserve)}
}
func (m *hashmap) delete(k hashable) {
	if m != nil {
		hash := k.hash(m.keyType)
		head := m.table[hash]
		if head != nil {
			if k.eq(m.keyType, head.key) {
				m.table[hash] = head.next
				m.length--
				return
			}
			prev := head
			for e := head.next; e != nil; e = e.next {
				if k.eq(m.keyType, e.key) {
					prev.next = e.next
					m.length--
					return
				}
				prev = e
			}
		}
	}
}