In Custom Command section of boost-build documentation there are 3 ways to use custom command:
* using make rule
* using notfile rule
* using generate rule

But all of these ways make only one target.

If project has some procedure that generates many source files at once, how can I declare them?

Example:

program make_sources creates a1.cpp and a2.cpp
a1.cpp and a2.cpp are not available  before build.

main target is
exe a : a1.cpp a2.cpp ;

How to declare a1.cpp and a2.cpp targets?