From v2.1, CasADi can be used as an alternative to GPOPS-II and ADiGator. CasADi is an open-source tool for nonlinear optimization and algorithmic differentiation (https://web.casadi.org/). Results using CasADi and GPOPS-II are very similar (differences can be attributed to the different direct collocation formulations and scaling). We used CasADi's Opti stack, which is a collection of CasADi helper classes that provides a close correspondence between mathematical NLP notation and computer code (https://web.casadi.org/docs/#document-opti). CasADi is actively maintained and developed, and has an active forum (https://groups.google.com/forum/#!forum/casadi-users).