in supports string contains. For example, 'foo' in 'foobar' == true

in supports collection contains. For example, 'foo' in [ 'foo', 'bar' ] == true

in supports map key contains. For example, 'foo' in { 'foo' : 5} == true

not in is the negation of the in expression. For example, 'grok' not in 'foobar' == true`