-If DEPTH is nonzero, paren depth begins counting from that value,
-only places where the depth in parentheses becomes zero
-are candidates for stopping; COUNT such places are counted.
-Thus, a positive value for DEPTH means go out levels.
+If DEPTH is nonzero, treat that as the nesting depth of the starting
+point (i.e. the starting point is DEPTH parentheses deep). This
+function scans over parentheses until the depth goes to zero COUNT
+times. Hence, positive DEPTH moves out that number of levels of
+parentheses, while negative DEPTH moves to a deeper level.