S swaps sign of literal, shift + s swaps sign mode of readout

main.go

@@ -213,6 +213,16 @@ func onPress (button stone.Button, modifiers stone.Modifiers) {
 		application.Draw()
 
 	case 's':
+		switch selectedExpression.(type) {
+		case *IntegerLiteral:
+			integer := selectedExpression.(*IntegerLiteral)
+			integer.value *= -1
+		}
+		
+		redraw()
+		application.Draw()
+
+	case 'S':
 		showSigned = !showSigned
 		redraw()
 		application.Draw()