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

parser_test.go

@@ -128,6 +128,9 @@ func TestParser(t *testing.T) {
 		/* 101 */ {`false or (x==2)`, nil, errors.New(`undefined variable "x"`)},
 		/* 102 */ {`a=5; a`, int64(5), nil},
 		/* 103 */ {`a=5; b=2; add(a, b*3)`, int64(11), nil},
+		/* 104 */ {`2=5`, nil, errors.New(`assign operator ("=") must be preceded by a variable`)},
+		/* 105 */ {`2+a=5`, nil, errors.New(`left operand of "=" must be a variable`)},
+		/* 106 */ {`2+(a=5)`, int64(7), nil},
 	}
 	succeeded := 0
 	failed := 0