Introduced in 704cfdaf0893fa4078ab744a915a56367282dcaa of @wu-s-john 

g++ compiler error: 

Pooling.cpp:177:13: error: explicit specialization in non-namespace
scope ‘class

Design looks good, g++ is just freaking out for no good reason.
Un-nesting the template classes fixes the error.

We don't have g++ CI. This hopefully happens infrequently enough that we
can just fix manually. My service to those folks who really like
building with g++... :)