Need help to interpret unusual Maxima notation for a derivative i.e. (d^{y+1}/dx^{y+1}) rho

Hello Martin,
 
You are absolutely right.
 
The short hand for a single derivative df/dx = diff(f,x) instead of diff(f,x,1) doesn't carry over for mixed partial derivatives i.e. Maxima doesn't assume a default argument of 1 so one has to type in the order of derivatives followed by the next derivative argument and that respective order of derivative etc.
 
Just tried it now and it works fine with no problems.
 
My next challenge is once I set up the finite difference scheme for the underlying PDE's is to write the m-files in Gnu Octave to solve. 
 
Probably the easiest way to do this is to use the subst function on e.g. eq = df/dx
 
eq_new: subst((f[i+1] - f[i])/([x[i+1] - x[i]),'diff(f,x),eq)
 
and then apply the fortran command
 
eq_octave: fortran(eq_new);
 
and just replace the square brackets with round brackets in Notepad++ say.
 
The only catch is that I might have to do this "manually" because I not sure how to iterate over a list of variables in Maxima e.g. {rho, u, x} with something like a for loop which only works with numbers.
 
Kind regards,
 
Vishal