extract_joint: Helper function for extracting joint distributions at...
In benkeser/intermed:

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

1	extract_joint(Q_M_n_i, M1_i, M2_i, all_mediator_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.
`M1_i`	The particular value of `M1` at which you are interested in evaluating the joint density. Note that the general idea is to `mapply` over this function to extract the joint under each observed value of `C` and extract the marginal distribution at the corresponding value of `M1` and `M2`.
`all_mediator_values`	All possible combinations of `M1` and `M2`. Used to identify which entry in `Q_M_n_i` that corresponds to values specified in `M1_i` and `M2_i`.
`a_val`	Which treatment are you interested in extracting the marginal under? Possible values are `"a_star"` and `"a"`
`...`	Not used