I not in Reals should be True (fix #7680)
added test for issue 6157
Merge pull request #7690 from hargup/issue6157_test

Test for issue 6157
Merge pull request #7683 from skirpichev/7680-I-not-in-Reals

I not in Reals should be True
added reference for ImageSet.intersect

    The implementation was derived by the Richard Fateman's paper
    http://www.cs.berkeley.edu/~fateman/papers/sets.pdf
Merge pull request #7693 from hargup/add_intersection_reference

added reference for ImageSet.intersect