;Given a variable of any type, check if it is a single valid pointer, and if
;so, keep dereferencing it until it's not a single valid pointer anymore. 
;Safe to use on any variable of any type. This will only return something
;different if the input variable is a single valid pointer.
;
;example:
; IDL> h=ptr_new(4)
; IDL> print,dereference_single(h)  ;same as print,*h
;        4
; IDL> h=ptr_new(ptr_new(ptr_new(h)))
; IDL> print,dereference_single(h)  ;same as print,*(*(*(*h))))
;        4
;Note that the input parameter is not modified in any way.
function dereference_single,a
  return,snap(a)
end