Return this block's hash. Uses a cached value if one exists. If you change the block be sure to call calcHash() or invalidateHash() to update this value either greedily or lazily respectively
Force recalculation of hash. To access the hash just use #hash, to force recalc call "changed()"
Communicate that this object has been changed, so recalculation of cached data is needed
run checks (such as verifying POW claim) & return true if the block header is consistent