j=[0...3]
M    ╒═══╕     ╒═══╕┌────┬────┬────┬────┐
└───>│ 2 │  ┌─>│ 4 ││ 10 │ 11 │ 12 │ 13 │
     ╞═══╡  │  ╘═══╛└────┴────┴────┴────┘
i==0 │ *─┼──┘          j=[0...5]              M[1][6─1]
     ├───┤     ╒═══╕┌────┬────┬────┬────┬────┬────┐
i==1 │ *─┼────>│ 6 ││ 14 │ 15 │ 16 │ 17 │ 18 │ 19 │
     ├───┤     ╘═══╛└────┴────┴────┴────┴────┴────┘
i==2 │ *─┼──┐          j=[0...2]
     └───┘  │  ╒═══╕┌────┬────┬────┐
            └─>│ 3 ││ 20 │ 21 │ 22 │
               ╘═══╛└────┴────┴────┘
             M[2].length

/** Muestra que las filas de una matriz pueden ser de diferente tamaño. */
/*
                       j=[0...3]
M    +---+     +---++-------------------+
+--->¦ 2 ¦  +->¦ 4 ¦¦ 10 ¦ 11 ¦ 12 ¦ 13 ¦
     ¦---¦  ¦  +---++-------------------+
i==0 ¦ *-+--/          j=[0...5]              M[1][6-1]
     +---¦     +---++-----------------------------+
i==1 ¦ *-+---->¦ 6 ¦¦ 14 ¦ 15 ¦ 16 ¦ 17 ¦ 18 ¦ 19 ¦
     +---¦     +---++-----------------------------+
i==2 ¦ *-+--\          j=[0...2]
     +---+  ¦  +---++--------------+
            +->¦ 3 ¦¦ 20 ¦ 21 ¦ 22 ¦
               +---++--------------+
             M[2].length
*/
static void main_matriz_loca() {
    boolean assertTrue = false;
    int M[][] = new int[2][];     assertTrue = ( 2 == M.length );
    M[0] = new int[4];            assertTrue = ( 4 == M[0].length );
    M[1] = new int[6];            assertTrue = ( 6 == M[1].length );
    M[2] = new int[3];            assertTrue = ( 3 == M[2].length );

    int n = 10;
    for (int i=0; i<M.length; ++i ) {
        for (int j=0; j<M[i].length; ++j ) {
            M[i][j] = n++;
        }
    }
    assertTrue = ( 10 == M[0][0] );    assertTrue = ( 14 == M[1][0] );
    assertTrue = ( 11 == M[0][1] );    assertTrue = ( 19 == M[1][5] );
    assertTrue = ( 21 == M[1][1] );
}