Adding interface Constraint{}.

error.go

@@ -47,6 +47,7 @@ var (
 	ErrUnsupportedDestination  = errors.New(`Unsupported destination type.`)
 	ErrUnsupportedType         = errors.New(`This type does not support marshaling.`)
 	ErrUnsupportedValue        = errors.New(`This value does not support unmarshaling.`)
+	ErrUnknownConditionType    = errors.New(`Arguments of type %s can't be used as constraints.`)
 )
 
 // Deprecated but kept for backwards compatibility. See: https://github.com/upper/db/issues/18

main.go

@@ -330,8 +330,14 @@ type Unmarshaler interface {
 	UnmarshalDB(interface{}) error
 }
 
-// ID is the interface implemented by structs that can set its own ID after
+// ID is the interface implemented by structs that can set their own ID after
 // calling Append().
 type ID interface {
 	SetID(...interface{}) error
 }
+
+// Constraint is the interface implemented by structs that can delimit
+// themselves.
+type Constraint interface {
+	Constraint() Cond
+}