src/main/java/org/organicdesign/fp/xform/Xform.java
@Override public <B> B fold(B ident, Fn2<? super B,? super A,B> reducer) {
        if (reducer == null) {
            throw new IllegalArgumentException("Can't fold with a null reduction function.");
        }

        // Construct an optimized array of OpRuns (mutable operations for this run)
        RunList runList = toRunList();
        return _fold(runList, runList.opArray(), 0, ident, reducer);
    }
@Override public <B> B fold(B ident, Fn2<? super B,? super A,B> reducer) {
        if (reducer == null) {
            throw new IllegalArgumentException("Can't fold with a null reduction function.");
        }

        // Construct an optimized array of OpRuns (mutable operations for this run)
        RunList runList = toRunList();
        return _fold(runList, runList.opArray(), 0, ident, reducer);
    }
Provides a way to collect the results of the transformation.