Many users have discovered to their dismay that when switching from
ordinary to the amsmath package, they are no longer able
to use abbreviations such as || || for |
|. This has to do with unfortunately nontrivial technical
complications: the environments such as align must read their
contents as a delimited macro argument because they do multipass
processing of the contents using algorithms inherited from Spivak's
amstex.tex. The obvious solution—substitution of different
algorithms that do box shuffling instead of token shuffling for the
multipass calculations—would require rewriting these display
environments from the ground up; while that is a worthy goal, it was
beyond the original scope of the / project. Some progress has
in fact been made on such a solution [time of writing: January 1995],
but not yet to the point of being ready for release.