That is , formula _ 144 is in formula _ 145 , provided formula _ 146 for some formula _ 147 . Observe that formula _ 56 and formula _ 57 can be disjoint and yet formula _ 145 can be nonempty .
That is , formula 144 is in formula 145 , formula 145 formula 146 for some formula 147 . Observe that formula 56 and formula 57 can be disjoint and yet provided can be nonempty .