b = isLinear M
Note that a linear matrix, in this sense, can still have different target degrees (in which case the cokernel decomposes into a direct sum by generator degree.)
The object isLinear is a method function.
The source of this document is in /build/reproducible-path/macaulay2-1.25.05+ds/M2/Macaulay2/packages/CompleteIntersectionResolutions.m2:3463:0.