src/main/java/com/basistech/tclre/Runtime.java
private void subset(RuntimeSubexpression sub, int begin, int end) {
        int n = sub.number;

        assert n > 0;

        while (match.size() < (n + 1)) {
            match.add(null);
        }

        match.set(n, new RegMatch(begin, end));
    }
private void subset(RuntimeSubexpression sub, int begin, int end) {
        int n = sub.number;

        assert n > 0;

        while (match.size() < (n + 1)) {
            match.add(null);
        }

        match.set(n, new RegMatch(begin, end));
    }
subset - set any subexpression relevant to a successful subre