[1 of 3] Processing p
  [1 of 2] Compiling A[sig]           ( p/A.hsig, nothing )
  [2 of 2] Compiling C[sig]           ( p/C.hsig, nothing )
[2 of 3] Processing q
  [1 of 2] Compiling B[sig]           ( q/B.hsig, nothing )
  [2 of 2] Compiling C[sig]           ( q/C.hsig, nothing )
[3 of 3] Processing r
  [1 of 5] Compiling H2[sig]          ( r/H2.hsig, nothing )
  [2 of 5] Compiling H1[sig]          ( r/H1.hsig, nothing )
  [3 of 5] Compiling H3[sig]          ( r/H3.hsig, nothing )

bkpfail21.bkp:1:1: error: [GHC-93009]
    • While merging export lists, could not unify {H1.T} with {H2.T}
      Neither name variable originates from the current signature.
    • while merging the signatures from:
        • p[A=<H1>,C=<H3>]:C
        • q[B=<H2>,C=<H3>]:C
        • ...and the local signature for H3