The margin control format
{ i}
( pushm) where
pushes a new left margin i characters to the right of the
format position onto the margin stack. The layout algorithm
uses the top of the margin stack to decide the column to start laying
out at after a line break.
The margin control format { 
}
( popm)
pops the current margin off the top of the margin stack and restores
the left margin to a previous margin.
Usually display forms should have matching pushm's and popm`s.