underscores in debug output?

[Christopher Howard]

Hi, what does the underscore character mean in this debug output?

   159:37  5 (delete ((3 . "c") (((2 . "b") (()) 1) (5 . "e") (()) 1)
2) 2)
   163:49  4 (delete ((2 . "b") (()) 1) _)
    152:9  3 (delete _ _)
    86:11  2 (skew _)
     64:6  1 (horizontal-left-link? _)
    46:11  0 (left-tree _)

Knowing the value of those parameters would be super helpful here, but
for some reason they have been replaced by underscores.