Better Clarity and COntrol on AND and OR for Advanced search

When using the advanced search tab to find contacts, it is not consistent whether "and" or "or" are used for search criteria.

For example, if I check two groups, A and B, any contact that is a member of A OR B or both is returned.

However, if I select group A and college Harvard I will get any person who is a member of group A AND went to Harvard.

The most important thing is to document how the search program works, i.e. when it uses AND and when it uses OR. Even better is to allow the user to control whether AND or OR is used. Currently this can be done by either sequentially creating a group or tag and assigning people to it, then doing a second search for the people who should be excluded and deleting records from the first group (AND) or by creating a group with an advanced search and then doing a second advanced search and then assigning the results to the first group (OR).