Illegal equational constraint v ~ Identifier

[jwaldmann]

this precludes building of docs on hackage,

https://hackage.haskell.org/package/tpdb-2.7.2/reports/2

[12 of 31] Compiling TPDB.Convert     ( src/TPDB/Convert.hs, dist/build/TPDB/Convert.o, dist/build/TPDB/Convert.dyn_o )

src/TPDB/Convert.hs:25:12: error:
    " Illegal equational constraint v ~ Identifier
      (Use GADTs or TypeFamilies to permit this)
    " In the type signature:
        trs2srs :: (Eq v, TermC v s, v ~ Identifier) =>
                   TRS v s -> Maybe (SRS s)