public FunctionParser(ExpressionParser expressionParser)
 {
     _expressionParser = expressionParser;
 }
 public FactorParser(ExpressionParser expressionParser, VariableParser variableParser, FunctionParser functionParser)
 {
     _expressionParser = expressionParser;
     _variableParser   = variableParser;
     _functionParser   = functionParser;
 }