src/main/java/org/organicdesign/fp/collections/RangeOfInt.java
@Override public int indexOf(Object o) {
        if (o instanceof Number) {
            int i = ((Number) o).intValue();
            if ( (i >= start) && (i < end) ) {
                return i - start;
            }
        }
        return -1;
    }
@Override public int indexOf(Object o) {
        if (o instanceof Number) {
            int i = ((Number) o).intValue();
            if ( (i >= start) && (i < end) ) {
                return i - start;
            }
        }
        return -1;
    }
Unlike most implementations of List, this method has excellent O(1) performance!
{@inheritDoc}