parser: now checks whether the assign operator '=' is preceded by a variable

parser.go

@@ -76,6 +76,7 @@ func (self *parser) parseList(scanner *scanner) (tree *term, err error) {
 func (self *parser) parse(scanner *scanner, termSymbols ...Symbol) (tree *ast, err error) {
 	tree = NewAst()
 	firstToken := true
+	lastSym := SymUnknown
 	for tk := scanner.Next(); err == nil && tk != nil && !tk.IsTerm(termSymbols); tk = scanner.Next() {
 		if tk.Sym == SymComment {
 			continue
@@ -112,9 +113,16 @@ func (self *parser) parse(scanner *scanner, termSymbols ...Symbol) (tree *ast, e
 			if listTerm, err = self.parseList(scanner); err == nil {
 				err = tree.addTerm(listTerm)
 			}
+		case SymEqual:
+			if lastSym == SymIdentifier {
+				err = tree.addToken(tk)
+			} else {
+				err = fmt.Errorf(`assign operator (%q) must be preceded by a variable`, tk.source)
+			}
 		default:
 			err = tree.addToken(tk)
 		}
+		lastSym = tk.Sym
 	}
 	return
 }