FRE - a memory management function that returns the amount of
      available memory
 
Syntax
  FRE(stringexpression)
  FRE(numeric-expression)
    - stringexpression, which can have any value, indicates to the
      function that you want free string memory compacted into a single
      block and then the size, in bytes, of that block to be returned.
    - numeric-expression, which has the value of -1, -2, or any other
      numeric value, indicates what type of status information you want
      returned about string or stack space (see <DETAILS>).