aHeader method
Lay String in a certain place in matrix. :QUEST: This is too complicated, lol.
Implementation
void aHeader(int _r, _c, String s) {
String _s1 = _matrix[_r].substring(0, _c);
int _pos = _s1.length;
int _length = _matrix[_r].length;
String _s3 = _matrix[_r].substring(_pos + s.length, _length);
_matrix[_r] = '$_s1$s$_s3';
}