"filter" statement
------------------

similar usecases as lambdas in other programming languages.

example:

    filter E^ entry in thelist {
        if entry^.name == none  exclude;
        if strcmp(entry^.name, name) == 0  include;
        exclude;
    }

equivalent to:

    {
        L^ tmplist = L:new();
        for E^ entry in thelist {
            if entry^.name == none  continue;
            if strcmp(entry.^name, name) == 0 goto include;
            continue;
          include:
            tmplist->add(entry);
        }
    }