[texhax] A useful macro?

[Michael Barr]

Could someone design a macro, say call it \seq, such that any comma inside 
the parameter of \seq would expand to ,\, .  So that \seq{x,y,z} would 
give x,\,y,\,z, \seq{x_1,x_2,\ldots,x_n} would give 
x_1,\,x_2,\,\ldots,\,x_n , and \seq{x_1,x_2,x_3,\ldots} would give 
x_1,\,x_2,\,x_3,\,\ldots .  You could try giving it mathcode 8000 and 
defining it inside math mode to give ,\,  but there is no way to just have 
an ordinary comma then.

Michael Barr