%
% execute the next closure step, if any
%
[=Closure]
[Any?]
{
%
% execute the next closure command
% this command may alter closure stack, add execs or remove execs
% command may delay closures until an event occurs because
% it is also responsible for initiating another closing step
%

 [PopLast] execstr

}
{
%
% no more exiting steps, desired closures reached
%
} ifelse