Sstrhsrtj

I would like to enclose multiple lines in an align in a group.
Because of the group, the \ delimiting the rows generates a mistake (if I'm guessing right the group is broken at \ and latex complains about unclosed groups).

My goal is to have a macro mymath that typesets some complex formula.
The formula is a combination of fixed symbols and optional symbols that are defined via local macros (A and B) in the MWE.
I would like the definitions to be local to the arguments of the mymath macro,
but I would also like the ability to break lines and align content from "outside" the macro, i.e. without baking in align or similar environments inside the macro itself.

MWE

documentclass[12pt]{article}

usepackage{amsmath}

defA{alpha_0}defB{beta_0}

defmymath#1#2{defA{alpha}defB{beta}sum #1=[#2]}

defmymathb#1#2{defA{gamma}defB{delta}sum #1=[#2]}

defGmymath#1#2{{defA{alpha}defB{beta}sum #1=[#2]}}

defGmymathb#1#2{{defA{gamma}defB{delta}sum #1=[#2]}}



begin{document}

begin{equation}

 mymath{A}{B}

 +A % want alpha_0 here

 mymathb{A}{B}

end{equation}

color{red}

begin{align*}

 mymath{A&}{5B}\

 mymathb{A&}{7B\&qquadA}

end{align*}

color{blue}

begin{multline*}

 mymath{A B}{long+\B superlong A}

end{multline*}

end{document}

If I use mymath the definitions of A and B are lost once I use \, and are not local to the argument only.

If I use gmymath the extra group make align and friends very confused about the \.

Question: Is there a way in which mymath can make the \ "leak" the group?
The same story applies to alignment characters &.