@Override
    protected E applyStringOperation(String item) {
        return (E) item.toLowerCase();
    }
Reference implementation for the local scope of the toUpper() step, a mid-traversal step which returns an upper case
string representation of all elements inside the incoming list traverser. Null values are not processed and remain as
null when returned. If the incoming traverser is not a list or a non-String list then an {@code IllegalArgumentException}
will be thrown.

@author David Bechberger (http://bechberger.com)
@author Yang Xia (http://github.com/xiazcy)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/ToLowerLocalStep.java