public boolean isStructural() {
      return this == STRUCTURAL
          || this == STRUCTURAL_IMPLEMENTS;
    }
Means object hashcode is cached at some point. Either lazily (on first access)
or eagerly (during construction).
@see #isUseLazyhash()
@see #isUsePrehashed()
value-processor/src/org/immutables/value/processor/meta/ValueType.java