This function opens up a hole in this, fills it in with attributes from the left, and then merges
on top of the hole.
This operation is equivalent to stretching every attribute that applies at position
pos in this
by an amount
len, and then calling change with a copy of each
other in sequence (offset in position by
This operation proves useful for, for instance, inserting a pre-edit string in the middle of an edit buffer.
the position in this at which to insert
the length of the spliced segment. (Note that this must be specified since the attributes in