isSubquotient(Module,Module) -- check whether a module is a subquotient of another

i1 : R = QQ[a..d];

i2 : N = coker matrix{{a,b},{c,d}}

o2 = cokernel | a b |
              | c d |

                            2
o2 : R-module, quotient of R

i3 : N1 = N/(a^4*N)

o3 = cokernel | a4 0  a b |
              | 0  a4 c d |

                            2
o3 : R-module, quotient of R

i4 : M = a*N/(R*a*N_0+a*b*N)

o4 = subquotient (| a 0 |, | 0  ab 0  a b |)
                  | 0 a |  | -c 0  ab c d |

                               2
o4 : R-module, subquotient of R

i5 : isSubquotient(M,N)

o5 = true

i6 : isSubquotient(M,N1)	  

o6 = false