17.7 The example heuristic function for the blocks world developed in this chapter simply counts the goals to be achieved. This is very crude as some goals are clearly more difficult than others. For example, it is trivial to achieve on

(a,

b) if blocks a and b are already clear, whereas it is difficult if a and b are buried under high stacks of other blocks. Therefore, a better heuristic function would try to estimate the difficulty of individual goals - for example, take into account the number of blocks to be removed before the block of interest could be moved. Propose such better heuristic functions and experiment with them.