Built-In Preprocessor for context arguments

Discussion
23

It’s easy to add a “context” parameter using package(edcg):

:- use_module(library(edcg)).
:- multifile edcg:pass_info/1.

edcg:pass_info(context).
edcg:pred_info(my_pred, 2, [context]).

my_pred(X, Y) -->>
    Context/context,  % Unify Context with the `context` passed argument
    do_something(X, Y, Context.some_field).

This expands my_pred/2 into my_pred/3:

my_pred(X, Y, Context) :-
    do_something(X, Y, Context.some_field).
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