Hello everybody,

I am studing the book of Paul Wellin and I am trying to solve a problem. But I am stuck, This is the question
Using ReplaceList write a function 'cartesianproduct' that works with two lists givings

CartPr[{{x1,x2},{y1,y2}}] = {{x1,y1},{x1,ÿ2} ,{x2,y1} ...}

first tried directly and that works

In

ReplaceList[{{x1, x2, x3}, {y1,   y2}}, {{___, a_, ___} , {___, b_, ___}} -> {{a, b}}]

Out[11]= {{{x1, y1}}, {{x1, y2}}, {{x2, y1}}, {{x2, y2}}, {{x3, 
   y1}}, {{x3, y2}}}

and now the function that doesn't work

cartesianProduct[{x_List, y_List}] = 

ReplaceList[{x, y}, { , a, ___ } , { , b, ___ } -> {a, b}]

ReplaceList::innf: Non-negative integer or Infinity expected at position 3 in ReplaceList[{x,y},{,a,},{,b,}->{a,b}]. >>

What do I do wrong ?

Thanks