This implements a sort of "safety net", adding some compile and runtime
error checking and reporting.
The first of these is to extend the @ and ! sigils to check for
the existence of a word name. In a basic image, if the name isn't found,
it will resolve to address 0 silently. This will have it report an error
if the word is not found.
~~~:err:var-not-defined
'\nERROR:_variable_%s_not_defined\n s:format s:put bye ;
:if:not-defined (sq-s)
over d:lookup n:zero? swap if ;
:sigil:@
[ err:var-not-defined ] if:not-defined
d:lookup d:xt fetch class:data |fetch ; immediate
:sigil:!
[ err:var-not-defined ] if:not-defined
d:lookup d:xt fetch class:data |store ; immediate
~~~