@Override
    public IntStream mapToInt(LongToIntFunction mapper) {
        requireNonNull(mapper);
        if (STRICT) {
            return toStream().mapToInt(mapper);
        }
        return SingletonIntStream.of(mapper.applyAsInt(element));
    }
An implementation of a LongStream that takes exactly one element as its
source.

This implementation supports optimized implementations of most terminal
operations and a some number of intermediate operations. Un-optimized
operations just returns a wrapped standard LongStream implementation.

For performance reasons, the LongStream does not throw an
IllegalStateOperation if methods are called after a terminal operation has
been called on the Stream. This could be implemented using a boolean value
set by each terminating op. All other ops could then assert this flag.

@author Per Minborg
@since  1.0.0
common-parent/singletonstream/src/main/java/com/speedment/common/singletonstream/SingletonLongStream.java