# Function: CallbackChain

## CALL:

CallbackChain(obj, field, callback, priority)

## DESCRIPTION:

Add an extra callback to an object. This is for example useful in combination with zoomtool

## INPUT:

obj | any |
    handle of object to which an extra callback has to be added.
field | any |
    string with fieldname of callback, e.g. 'keypressfcn'.
callback | any |
    Any valid callback, that is:
        String (for eval)
        Function pointer (for feval)
        Cell array, where element 1 is function pointer
priority | any |
    'first', {'last'}, 'drop'
    if 'first' put this callback first in the chain.
    if 'last' put this callback last in the chain.
    if 'drop' (Not implemented yet!!)  find callback function with
    specific signature and if present, remove it from the callback
    chain.

## OUTPUT:

         no direct output

      EXAMPLE 1: