getting absolute path of a file in csh?

[Robert La Ferla]

In a csh script, how do you get the absolute path of a filename?

e.g.

If the cwd is "/users/myacct" and there is a file called  
"filename.dat", I want a command (built-in or external)  that given  
the argument "filename.dat", returns "/users/myacct/filename.dat".   
However, if given "/users/myacct/filename.dat" as the input argument,  
it should also give back "/users/myacct/filename.dat"