`Variable.t`s represent variables than can be used as predictors or the responder when specifying a regression.

`type t` = `[`

`|` ``Runs`
`|` ``Cycles`
`|` ``Nanos`
`|` ``Compactions`
`|` ``Minor_collections`
`|` ``Major_collections`
`|` ``Promoted`
`|` ``Minor_allocated`
`|` ``Major_allocated`
`|` ``One`

`]```
`include sig ... end`
`val t_of_sexp : Base.Sexp.t ‑> t`
`val __t_of_sexp__ : Base.Sexp.t ‑> t`
`val sexp_of_t : t ‑> Base.Sexp.t`