In past versions of Analytic Solver, if a user was unable to calculate a value using Excel's function library, he/she were driven to define their own VBA function. However, such functions were not supported by the PSI Interpreter; a severe limitation. In the latest version of Analytic Solver, this limitation has been overcome with the implementation of custom Box functions and the support of Excel's new LAMBDA function. A Box function is invoked using the Psi function, PsiBoxFunction().