extract_marginal: Helper function for extracting marginal distributions at...
In benkeser/intermed:

Helper function for extracting marginal distributions at particular values of the mediator.

extract_marginal(
  Q_M_n_i,
  mediator = "M1",
  M_i,
  unique_M_values,
  a_val = "a_star",
  ...
)

`Q_M_n_i`	An entry in the `Q_M_n` list, where the first entry in the list corresponds to mediator distributions under `a_star`; the second entry in the list corresponds to mediator distributions under `a`. In each of these lists, we have the joint of M1,M2; the marginal of M1; and the marginal of M2.
`mediator`	Which mediator are you interested in extracting the marginal of? Possible values are `"M1"` and `"M2"`.
`M_i`	The particular value at which you are interested in evaluating the marginal density. Note that the general idea is to `mapply` over this function to extract the marginal under each observed value of `C` and extract the marginal distribution at the corresponding value of e.g., `M1`.
`unique_M_values`	The unique values that the selected `mediator` can assume. Recall that the ordering of the marginal entries in `Q_M_n_i` is in this order.
`a_val`	Which treatment are you interested in extracting the marginal under? Possible values are `"a_star"` and `"a"`
`...`	Not used