|  |  4.12.3 matrix type cast 
See
 Type conversion and casting;
 intmat type cast;
 matrix.Syntax:matrix (expression)
 matrix (expression, int_n, int_m)Type:matrix
Purpose:Converts expression to a matrix, where expression must be of type int,
intmat, intvec, number, poly, ideal, vector, module, or matrix. If
int_n and int_m are supplied, then they specify the dimension of the
matrix. Otherwise, the size (resp. dimensions) of the matrix
is determined by the size (resp. dimensions) of the
expression.
Example:|  |   ring r=32003,(x,y,z),dp;
  matrix(x);
==> _[1,1]=x
  matrix(x, 1, 2);
==> _[1,1]=x
==> _[1,2]=0
  matrix(intmat(intvec(1,2,3,4), 2, 2));
==> _[1,1]=1
==> _[1,2]=2
==> _[2,1]=3
==> _[2,2]=4
  matrix(_, 2, 3);
==> _[1,1]=1
==> _[1,2]=2
==> _[1,3]=0
==> _[2,1]=3
==> _[2,2]=4
==> _[2,3]=0
  matrix(_, 2, 1);
==> _[1,1]=1
==> _[2,1]=3
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