I'm pretty sure it's a scoping issue. Your inner and outer arg1's should actually bind at different times to different objects, and the only way to signify that is to use scope the inner arg1 in a phx::lambda construct.

I don't remember exactly what you were trying to do, but I believe it was something like

phx::for_each(arg1, phx::push_back(dest, arg1))(srce)

which evaluates to approximately

for_each(srce, F)

where F(x) evaluates dest.push_back(srce) (regardless of x) every time, since the arg1 in phx::push_back(dest, arg1) is immediately bound to srce. phx::lambda effectively delays this binding to give you what you want:

phx::for_each(arg1, phx::lambda(phx::push_back(dest, arg1)))(srce)
  => for_each(srce, phx::push_back(dest, arg1))

Note that my syntax is way off here, but this scoping issue is present not just in Boost.Phoenix but also Boost.MPL and probably Boost.Bind/Std.Bind. Also consider reading up on de Bruijn indices on wikipedia; at some point in the past on this list, someone proposed using de Bruijn placeholders to avoid phx::lambda (and Phoenix local variables) acrobatics.

HTH,

- Jeff