public boolean isLocked() {
        return isLocked;
    }
Determine if modifications can still be made to the IteratorChain.
IteratorChains cannot be modified once they have executed a method
from the Iterator interface.

@return true if IteratorChain cannot be modified, false if it can
jphp-runtime/src/php/runtime/common/collections/iterators/IteratorChain.java