This button allows you to enter a new expression, or replace parts of an old
one.

To start off it is the only enabled button.

When pressed a requester will appear allowing you to enter a new expression.

Expressions are entered in fairly normal algebraic notation, but note

* Multiplication is denoted by `*', division by `/', exponentiation by `^'

* Multi letter variable names ARE allowed. Although they are entered all on
  one level they will be displayed as subscripts in the expression window.

* The `*' for multiplication can be missed out when no confusion would arrise
  for example:-
  
      OK: 2a  
          (a+b)(a-b)    
          2a(6c-3)

  NOT OK: 2ab           means 2 times a_subscript_b
          2a/b*c        means ( 2 times a divided by b ) all times c

* Brackets can be () or {}.
  The computer will automaticaly add/remove () brackets when appropriate.
  The computer will not remove {} brackets (exception - .

*  Functions are allowed, brackets are only needed where confusion could
  arrise. A function name must be followed by an open bracket or a space.