m = extendFromMiddle(F1,F2,f,i)
If f is a map to F1_i from F2_0, the script computes a ChainComplexMap to F1 from F2e, where F2e is a chain complex obtained from F2 by prepending zeros.
the process of making a new ChainComplex seems to destroy the direct sum information in the source and target modules.
The object extendFromMiddle is a method function.
The source of this document is in /build/reproducible-path/macaulay2-1.25.06+ds/M2/Macaulay2/packages/ChainComplexExtras.m2:1243:0.