POPUPWHEN()

  Short:
  ------
  POPUPWHEN() Allows use of a popup for a GET WHEN clause

  Returns:
  --------
  <lWhen> => when condition, get pre-validation

  Syntax:
  -------
  POPUPWHEN(bPopup,[lShowonUp],[lReturn])

  Description:
  ------------
  This sets up a popup in <bPopup> for use in a GET
  WHEN clause. (the pre validation block). If a value is returned
  from the popup, it is assigned to the get.

  <bPopup> is a codeblock that is evaluated. It is
  passed the current value in the get as a parameter.

  The popup function will pop up upon entry into a GET
  field.

  [lShowOnUp] Normally you wouldn't want this WHEN to
  occur if the user is using the up arrow, and if <lShowOnUp> is
  .f. (the default) it does not occur (the GET is just skipped)

  [lReturn] If set to False (the default) the GET is
  never actually edited, as the WHEN will always return .f., but it is
  assigned the value returned by the popup. By setting it to True,
  the Calendar will pop up, and then the GET will also be put
  through the normal get editor.

  Examples:
  ---------
   // these are codeblocks that do a popup (in this case, a  simple
   //call to msg, and a return of a value)

   b1 := {||msg("Character value"),"Bert"}
   b2 := {||msg("Logical value"),.t.}
   b3 := {||msg("Date value"),date()+100}

   v1 := space(10)
   v2 := .f.
   v3 := ctod("  /  /  ")

   @10,10 get v1 when POPUPWHEN(b1,.f.,.t.)
   @11,10 get v2 when POPUPWHEN(b2,.f.,.t.)
   @12,10 get v3 when POPUPWHEN(b3,.f.,.t.)

   READ

  Source:
  -------
  S_POPVW.PRG