--- This accumulates x in a where they are both nested tables of
--- tensors. If first is true, set a = x. Behavior is undefined if a
--- and x do not have the exact same structure.
+-- This accumulates x in a, where they are both nested tables of
+-- tensors with same structures / keys. If first is true, set a = x
+-- (in which case a can be nil) otherwise a = a + x. The behavior is
+-- undefined if a and x do not have the exact same structure.