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syzygyScheme(C,i,v) -- produce the syzygy scheme from a map v : R^j ---> C_i which selects some syzygies from a resolution C.

Code:

     -- ../../../Macaulay2/m2/chaincomplexes.m2:662-666
     syzygyScheme = (C,i,v) -> (
          -- this doesn't work any more because 'resolution' replaces the presentation of a cokernel
          -- by a minimal one.  The right way to fix it is to add an option to resolution.
          g := extend(resolution cokernel transpose (C.dd_i * v), dual C[i], transpose v);
          prune cokernel (C.dd_1  * transpose g_(i-1)))

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