fcn amb(f,a,b,etc){
   fcn(sink,f,a,b,etc){
      abc:=vm.arglist[2,*]; // ((the,that),(frog,elephant))
      if(abc.len()<2) return(sink.write(abc[0][0])); // back out of recursion
      foreach a,b in (abc[0],abc[1]){ // Cartesian product
	 if(f(a,b)) self.fcn(sink,f,T(String(a," ",b)),abc[2,*].xplode());
      }
   }(s:=List(),vm.pasteArgs());
   s
}