ISFIELD()

  Short:
  ------
  ISFIELD() Determines if an expression is the name of a field

  Returns:
  --------
  <lIsfield> => Is the expression a field

  Syntax:
  -------
  ISFIELD(cExpress)

  Description:
  ------------
  Determines if <cExpress> contains the name of a
  field. If <cExpress> contains an alias, the area of the alias
  will be checked.

  Examples:
  ---------
   isfield( "LASTNAME" )            // actual field, returns  .t.

   isfield( "CUSTOMER->LASTNAME")   // actual field, returns  .t.

   isfield( "LEFT(LNAME,5)" )       // expression, returns .f.

  Notes:
  -------
  Companion functions are ISTHISAREA(), ISEDITABLE(),
  PARSFIELD(), PARSALIAS(), EXPBLOCK(), WORKBLOCK()

  Source:
  -------
  S_FIELDS.PRG