code: function polydiff(symbol) {
                  if(symbol.value === d || symbol.contains(d, true)) { 
                      symbol.multiplier = symbol.multiplier.multiply(symbol.power);
                      symbol.power = symbol.power.subtract(new Frac(1)); 
                      if(symbol.power.equals(0)) {
                          symbol = Symbol(symbol.multiplier);
                      }
                  } 
                  
                  return symbol;
              }
docs: Equivalent to "derivative of the outside".