command: State based testing commands
In hedgehog: Property-Based Testing

This helper function assists one in creating commands for state machine testing in hedgehog.

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command(title, generator, execute, require = function(state, ...) T,
  update = function(state, output, ...) state, ensure = function(state,
  output, ...) NULL)

`title`	the name of this command, to be shown when reporting any failing test cases.
`generator`	A generator which provides random arguments for the command, given the current (symbolic) state. If nothing can be done with the current state, one should preclude the situation with a requires and return NULL. Otherwise, it should be a list of arguments (the empty list is ok for functions which take no arguments).
`execute`	A function from the concrete input, which executes the true function and returns concrete output. Function takes the (possibly named) arguments given by the generator.
`require`	A function from the current (symbolic) state to a bool, indicating if action is currently applicable. Function also takes the (possibly named) arguments given by the generator (this is mostly used in shrinking, to ensure after a shrink its still something which could have been generated by the function generator).
`update`	A function from state to state, which is polymorphic over symbolic and concrete inputs and outputs (as it is used in both action generation and command execution). It's critical that one doesn't "inspect" the output and input values when writing this function.
`ensure`	A post-condition for a command that must be verified for the command to be considered a success. This should be a set of testthat expectations.