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Add if statement

This is a first version of if, that it is not functional because
the direction of the jump is wrong. We have to negate
the condition in order to get the correct jump.

--- a/stmt.c

+++ b/stmt.c

@@ -260,6 +260,23 @@

 	lswitch->deflabel = ldefault;

 }

+static void

+If(Symbol *lbreak, Symbol *lcont, Caselist *lswitch)

+{

+	Symbol *end = label(NULL, 1);

+

+	expect(IF);

+	/* TODO: negate the condition */

+	emitjump(end, condition());

+	stmt(lbreak, lcont, lswitch);

+	if (accept(ELSE)) {

+		emitlabel(end);

+		stmt(lbreak, lcont, lswitch);

+	} else {

+		emitlabel(end);

+	}

+}

+

 void

 compound(Symbol *lbreak, Symbol *lcont, Caselist *lswitch)

 {

@@ -289,6 +306,7 @@

 	case WHILE:    While(lswitch); break;

 	case FOR:      For(lswitch); break;

 	case DO:       Dowhile(lswitch); break;

+	case IF:       If(lbreak, lcont, lswitch); break;

 	case BREAK:    Break(lbreak); break;

 	case CONTINUE: Continue(lcont); break;

 	case GOTO:     Goto(); break;