We plan to remove sys_pre_expand in OTP 20: https://github.com/erlang/otp/pull/1156 sys_pre_expand is an undocumented and unsupported module, but since it has been in Erlang/OTP since the beginning, we thought it better to give a heads up just in case. /Björn -- Björn Gustavsson, Erlang/OTP, Ericsson AB