Subscripting syntax error

I need to subscript set step in a parameter declaration but am getting an error message:

param torv{{sss in cartSR} step[sss]} binary ; 
	syntax error
context:  param torv{{sss in cartSR}  >>> step[ <<< sss]} binary ;

Please advise

You can write it like this:

set cartSR;
set step {cartSR};
param torv {sss in cartSR, st in step[sss]} binary;

Then in the model, the params are referenced as torv[sss,st].

For a detailed presentation of how this works, see section 6.5 Indexed collections of sets in the AMPL book.