Forward substitution solves a matrix equation in the form Lx = b
by an iterative process for a lower triangular matrix L.
The process is so called because for a lower triangular matrix, one first computes
then substitutes that forward into the next equation to solve for x2,
and repeats until xn.
Note that some diagonal entries in L can be 0s, provided that the system of equations is
0 & 0 & 0\\
2 & 0 & 0\\
4 & 5 & 6