cmp_str(u,v)=u==v
copy_str(v)=v \\ Creates a copy, not a pointer
append_str(v,n)=concat(v,n)
replace_str(source, n, replacement)=my(v=[]);for(i=1,#source,v=concat(v,if(source[i]==n,replacement,source[i]))); v

u=[72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100];
v=[];
cmp_str(u,v)
w=copy_str(v)
#w==0
append_str(u,33)
u[8..12]
replace_str(u,108,[121])
concat(v,w)