whole_func_string: function(ast) {
      let detected = [];
  
      ast.traverseByType('string', function(string) {
        if (string.content[0] === '"') {
          detected.push('double');
        } else if (string.content[0] === '\'') {
          detected.push('single');
        }
      });
  
      return detected;
    }
func_code_string: function(ast) {
      let detected = [];
  
      ast.traverseByType('string', function(string) {
        if (string.content[0] === '"') {
          detected.push('double');
        } else if (string.content[0] === '\'') {
          detected.push('single');
        }
      });
  
      return detected;
    }
func_documentation_string: Detects the value of an option at the tree node.
  
  @param {node} ast