Integer literals now support signs

main.go

@@ -164,7 +164,11 @@ func onPress (button stone.Button, modifiers stone.Modifiers) {
 		case *IntegerLiteral:
 			integer := selectedExpression.(*IntegerLiteral)
 			integer.value *= int64(integer.displayRadix)
-			integer.value += int64(value)
+			if integer.value < 0 {
+				integer.value -= int64(value)
+			} else {
+				integer.value += int64(value)
+			}
 		}
 		
 		redraw()