@Override
    default IndexColumn deepCopy() {
        return DocumentUtil.deepCopy(this, IndexColumnImpl::new);
    }
A typed {@link Document} that represents the column referenced by an index 
key instance in the database. An {@code IndexColumn} is located inside an
{@link Index}.

@author  Emil Forslund
@since   2.0.0
runtime-parent/runtime-config/src/main/java/com/speedment/runtime/config/IndexColumn.java