f = function (x) {
    scope.set(variable, x)
    return fnCode.evaluate(scope)
  }
A transformation for the integrate function. This transformation will be
invoked when the function is used via the expression parser of math.js.

Syntax:

    integrate(integrand, variable, start, end)
    integrate(integrand, variable, start, end, step)

Usage:

    math.evaluate('integrate(2*x, x, 0, 2)')
    math.evaluate('integrate(2*x, x, 0, 2, 0.01)')

@param {Array.<math.Node>} args
           Expects the following arguments: [f, x, start, end, step]
@param {Object} math
@param {Map} [scope]