fixup! fix: improve validation of variables in build_explicit_observed_function

Author: AayushSabharwal

src/utils.jl

@@ -1274,7 +1274,7 @@ end
 Check if `var` is present in `varlist`. `iv` is the independent variable of the system,
 and should be `nothing` if not applicable.
 """
-function var_in_varlist(var, varlist::Set, iv)
+function var_in_varlist(var, varlist::AbstractSet, iv)
     var = unwrap(var)
     # simple case
     return var in varlist ||