symbolic linear system solver
[x]=solve(A,b)
:A,b,x matrix (resp. vectors) of character strings :
solves A*x = b when A is an upper triangular matrix made of character strings.
A=['1','a';'0','2']; //Upper triangular
b=['x';'y'];
w=solve(A,b)
a=1;x=2;y=5;
`evstr`_(w)
`inv`_([1,1;0,2])*[2;5]