⁓ measure cost per trial

part of learn rate according to ❐ Using a Model to Compute the Optimal Schedule of Practice, citing (Balota et al., 2006; Cull, 2000; Cull et al., 1996; Landauer & Bjork, 1978; Rea & Modigliani, 1985).

actually needed (most of the time) if you want to ⁓ measure return on learning