Infrequent application of id rule

[jiwonparc]

The rule for "(par (A) (id A A))" might be enabled too infrequently as there are many other rules in the config rule. Not sure what would be the best way to deal with it, and how often it should be triggered; perhaps throw a random coin when we perform a mutation, say with probability p=0.1, we use it, otherwise, the other rules will be used. If p=1.0, it should be the same as what we have already.