====== Description ======
The subtraction / negation node either takes the difference of the values of its two child-nodes or returns the negation of its single child-node.

====== Constraints ======
  * Minimum number of child-nodes: 1
  * Maximum number of child-nodes: 2

====== Calculus ======
if number of child-nodes = 1 then value = -value<sub>1</sub>

if number of child-nodes = 2 then value = value<sub>1</sub> - value<sub>2</sub>