Implemented expression parsing

src/errors/KeywordNotImplemented.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// KeywordNotImplemented error is created when we find a keyword without an implementation.
+type KeywordNotImplemented struct {
+	Keyword string
+}
+
+// Error generates the string representation.
+func (err *KeywordNotImplemented) Error() string {
+	return fmt.Sprintf("Keyword not implemented: '%s'", err.Keyword)
+}