Play with mathematical formula

          SWAP sel is currently broken

          Selecting parts
          - red is selected
          - mouse click to select numbers or symbols

          Navigation
          - ESC to select parent of current selection
          - RIGHT ARROW select next child of current selection
          - LEFT ARROW select

          Changing math
          - +/- on number (rational or natural)

          Refactor
          - ^ on natural number or symbol raise to power 1
          - / on number divide by 1
          - + on non number add 0
          - * multiply by 1