Mat(E):MAT
Mat[E]:MAT -- obsolescent!

where E is either: a rectangular lists of lists, a vector, or a
module.

  Use R ::= QQ[x,y];
  L := [[1,2],[3,4]];
  Mat(L);
Mat([
  [1, 2],
  [3, 4]
])
-------------------------------
  M := Module([x,x^2,y],[x^2,y,0]);
  Mat(M);
Mat([
  [x, x^2, y],
  [x^2, y, 0]
])
-------------------------------
  Mat([[1,2],[3,4]]);
Mat([
  [1, 2],
  [3, 4]
])
-------------------------------
  Mat[[1,2],[3,4]];    -- only square brackets is obsolescent!
Mat([
  [1, 2],
  [3, 4]
])
-------------------------------
  -- a slightly more obscure example: allowed, but discouraged!
  M := Mat([["a","b"],["c",[1,2]]]);
  N := Mat([["d","e"],["f",[3,4]]]);
  M+N;
Mat([
  ["ad", "be"],
  ["cf", [4, 6]]
])
-------------------------------