When you insert text directly at the place where a marker points,
there are two possible ways to relocate that marker: it can point before
the inserted text, or point after it. You can specify which one a given
marker should do by setting its insertion type. Note that use of
insert-before-markers ignores markers' insertion types, always
relocating a marker to point after the inserted text.
This function sets the insertion type of marker marker to type. If type is
t, marker will advance when text is inserted at its position. If type is
nil, marker does not advance when text is inserted there.
This function reports the current insertion type of marker.
All functions that create markers without accepting an argument that
specifies the insertion type, create them with insertion type
nil (see Creating Markers). Also, the mark has, by
default, insertion type