Refine members data structure a bit.

selsl/value/value.go

@@ -2,7 +2,7 @@ package value
 
 import "fmt"
 
-type Members map[string]Value
+type Members map[string][]Value
 
 type Value interface {
 	Name() string
@@ -120,6 +120,8 @@ func (m Method) String() string {
 	return fmt.Sprintf("Method: %s", m.Name)
 }
 
+var _ Value = Error{}
 var _ Value = String{}
 var _ Value = Int{}
 var _ Value = List{}
+var _ Value = Method{}