Weight - Weight -- the difference of two weights

i1 : R=rootSystemA(4)

o1 = RootSystem{...8...}

o1 : RootSystem

i2 : M=cartanMatrix R

o2 = | 2  -1 0  0  |
     | -1 2  -1 0  |
     | 0  -1 2  -1 |
     | 0  0  -1 2  |

              4       4
o2 : Matrix ZZ  <-- ZZ

i3 : p1=weight(R,M_0)

o3 = |  2 |
     | -1 |
     |  0 |
     |  0 |

       4
o3 : ZZ

i4 : p2=weight(R,M_1)

o4 = | -1 |
     |  2 |
     | -1 |
     |  0 |

       4
o4 : ZZ

i5 : p1-p2

o5 = |  3 |
     | -3 |
     |  1 |
     |  0 |

       4
o5 : ZZ