father son - Please help me understand

[Forumer]

I've seen your example of forward chaining with tuples (great, grand) etc. But I don't understand what the following is doing with "father" and "son" without $ prefix. Please help me understand this or point me to where it is explained.

Thank you!

son_of

    foreach

family.son_of($child, $father, $mother)

    assert

family.child_parent($child, $father, father, son)

family.child_parent($child, $mother, mother, son)

[Forumer]

I think I get it. Although the order of 3rd and 4th arguments are not intuitive to me, it is what it is.