compress -- extract nonzero columns from a matrix

i1 : m = matrix {{1,2,0,0},{0,0,1,0}}

o1 = | 1 2 0 0 |
     | 0 0 1 0 |

              2        4
o1 : Matrix ZZ  <--- ZZ

i2 : compress m

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

              2        3
o2 : Matrix ZZ  <--- ZZ