public static SignedBlock Sign( Block block, Identity signature ){
		
		// remove it from the lookup
		uidLookup.remove(block.id);
		
		//now create new signed block
		SignedBlock sb = new SignedBlock( block.id, block.previous, signature );
	
		//now put it back in the lookup
		uidLookup.put(sb.id, sb);
		
		return sb;
		
	}