3C. types of expressions
------------------------
There are three types of expressions that may be used for different
purposes;

- <var>, consisting of just a variable
- <varexp>, consisting of a variable, possibly with unary operators
  with it, like "++" (increment) "." (member selection) or [] (array
  operator). (see  4D , 4G). It denotes a modifiable expression, like
  Lvalues in C.
  Note that those (unary) operators are not part of any precedence.
- <exp>. This includes <var> and <varexp>, and any other expression.