An interceptor is a `function(env)` consulted in [nlmixr2Est()] before the standard estimation method is dispatched. It receives the fully set-up estimation environment (`env$ui`, `env$data`, `env$control`, the est name in `class(env)[1]`, and any extra `nlmixr2()` arguments in `env$.nlmixr2Dots`). If it returns a non-`NULL` value that value becomes the fit – the interceptor has *claimed* the estimation; if it returns `NULL` it declines and the next interceptor (or the standard method) runs. While a claimed interceptor runs, interceptors are suppressed, so it may call `nlmixr2()` internally and get the ordinary methods.