4.5 How to find all csgn() and replace them by 1

I wanted to simplify an expression which could have csgn() in it, and find all the arguments.

\[ \frac {1+\mathrm {csgn} \left (a \right ) a}{3 \mathrm {csgn} \left (b \right ) b} \]

One way is

restart; 
expr:=(1+csgn(a)*a)/(3*csgn(b)*b): 
expr:=subsindets(expr,'specfunc( anything, csgn )',f->1);
 

\[ \frac {1+a}{3 b} \]