Sometimes it's necessary to line up a series of equations at some horizontal position, often at an equals sign. This is done with two operations called mark and lineup.
may appear once at any place in an equation.
It remembers the horizontal position where it appeared.
Successive equations can contain one occurrence of the word
The place where
appears is made to line up
with the place marked by the previous
if at all possible.
Thus, for example,
you can say
x+y mark = z
x lineup = 1
For reasons too complicated to talk about,
when you use
don't work with centered equations.
Also bear in mind that
doesn't look ahead;
x mark =1
x+y lineup =z
isn't going to work, because there isn't room for the x+y part after the mark remembers where the x is.