code: function(expr, context, subIntegral) {
        if(expr.type === "OperatorNode" && expr.op === '*') {
          var constantFactors = [];
          var nonConstantFactors = [];
          expr.args.forEach(function(expr) {
            if(context.isConstant(expr)) {
              constantFactors.push(expr);
            } else {
              nonConstantFactors.push(expr);
            }
          });
  
          if(constantFactors.length !== 0) {
            var constantNode = factorsToNode(constantFactors);
            var nonConstantNode = factorsToNode(nonConstantFactors);
  
            var nonConstantIntegral = subIntegral(nonConstantNode, context, "multiplication by constant");
            if(nonConstantIntegral) {
              return new OperatorNode('*', 'multiply', [constantNode, nonConstantIntegral]);
            }
          }
  
          function factorsToNode(factors) {
            if(factors.length === 1) {
              return factors[0];
            } else {
              return new OperatorNode('*', 'multiply', factors);
            }
          }
        }
      }
docs: integral(c*f(x), x) = c*integral(f(x), x)