set method
set
Implementation
void set(Body bi, Body bj, bool value) {
int i = bi.id;
int j = bj.id;
if (j > i) {
final temp = j;
j = i;
i = temp;
}
if (value) {
matrix['$i-$j'] = true;
} else {
matrix.remove('$i-$j');
}
}