-- -*- M2-comint -*- {* hash: -311109529 *}

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

i2 : F = (vars R) ++ ((vars R) ++ matrix{{a-1,b-3}})

o2 = | a b c d 0 0 0 0 0   0   |
     | 0 0 0 0 a b c d 0   0   |
     | 0 0 0 0 0 0 0 0 a-1 b-3 |

             3       10
o2 : Matrix R  <--- R

i3 : F_[1]

o3 = | 0 0 0 0 0   0   |
     | a b c d 0   0   |
     | 0 0 0 0 a-1 b-3 |

             3       6
o3 : Matrix R  <--- R

i4 : F_[1]^[1]

o4 = | a b c d 0   0   |
     | 0 0 0 0 a-1 b-3 |

             2       6
o4 : Matrix R  <--- R

i5 : N = (a=>vars R) ++ (b=>vars R)

o5 = | a b c d 0 0 0 0 |
     | 0 0 0 0 a b c d |

             2       8
o5 : Matrix R  <--- R

i6 : N_[a]

o6 = | a b c d |
     | 0 0 0 0 |

             2       4
o6 : Matrix R  <--- R

i7 : N = directSum(x1 => matrix{{a,b-1}}, x2 => matrix{{a-3,b-17,c-35}}, x3 => vars R)

o7 = | a b-1 0   0    0    0 0 0 0 |
     | 0 0   a-3 b-17 c-35 0 0 0 0 |
     | 0 0   0   0    0    a b c d |

             3       9
o7 : Matrix R  <--- R

i8 : N_[x1,x3]

o8 = | a b-1 0 0 0 0 |
     | 0 0   0 0 0 0 |
     | 0 0   a b c d |

             3       6
o8 : Matrix R  <--- R

i9 :