TermStruct is an other type that is unnecessarely complicated. This time because it has to mess around with the struct definition; however if we clean all tha complexity to what is strictly needed, TermStruct would look something like this:

type TermStruct<SDef extends ConstantableStructDefinition> = Term<PStruct<SDef>> & 
    IsSingleKey<SDef> extends true ?
        extract: ( ...fields: string[] ) => {
            in: <PExprResult extends PType>
                ( expr: ( extracted: StructInstance<...> ) => Term<PExprResult> )
                 => Term<PExprResult>
    : {}

even with these semplifications it might seem a bit complex but really all is telling us is that it adds the extract method only if the struct can only have one single constructor; and adds nothing if it has more.

Infact we already encountered this method while introducing pmatch; we just didn't know that it was an utility term.