code: function IntegrationContext(variable, options) {
      this.variable = variable.clone();
      this.options = options;
  
      this._constantExpr = {};
      this.subIntegral = {};
  
      this.rules = options.rules;
  
      this.debugIndent = 0;
    }
docs: @constructor IntegrationContext
  
  Encapsulates an integration context, including the following: the variable
  of integration; a memoizing isConstant function (to determine if a given
  expression is constant relative to the variable of integration); and a lookup
  table for already computed subintegrals.
  
  @param {SymbolNode} variable The variable of integration.
  @param {Object} options The integration options object.