I don't want to disable the function of reusing the solution of previous line (as requested in this issue: https://bitbucket.org/heldercorreia/speedcrunch/issues/389/automatic-insertion-of-ans-variable): I think it's a very useful feature.
But it can get confusing when you want to input a negative number at the start of the line: Typing: [-]  will results in : 'ANS-3'. If I want to avoid this, I could disable completely the feature of re-using the last result, or use brackets: [(] [-]  [)], which results in lot of extra-typing.
I suggest to implement a 'change sign' or 'negative number input' button. This feature exists in lots of different scientific calculator. Even the built-in iPhone calculator has this.
Then in addition we could create a custom shortcut to this button if we want to use a key on our PC keyboard.