This function is used to superimpose two line segment patterns
A and B.
The two patterns must have identical windows. If one
pattern has marks, then the other must also have marks
of the same type. It the marks are data frames then the
number of columns of these data frames, and the names of
the columns must be identical.
(To combine two point patterns, see superimpose).
If one of the arguments is NULL, it will be ignored
and the other argument will be returned.