is this a numpy expm1? It has problems in corner cases on some systems: https://github.com/numpy/numpy/issues/4225 (context and discussion: https://github.com/scipy/scipy/pull/3225); apparently `scipy.special.expm1` is better-behaved. Can you also add a test for corner cases, small `c` and `x`? I know, these corner cases are a hassle do deal with..

Ditto for `log1p` below.