Meta.FmtCustom formatter key.
The Fun.Generic.pp function consults this key in the metadata of a gist g before printing the value described by the gist. If a formatter is found, the value is formatted using that formatter instead. This can also be used to indicate that you would not like to print a substructure by formatting nothing, the Fmt.ignore function does that.
val ignore : 'a tignore formats nothing.
type 'a meta := 'a Type.Gist.Meta.tval mem : 'a meta -> boolmem m is true iff m has a binding for the key.