detect(ast) {
    var detected = [];

    ast.traverse(function(node) {
      // Continue only with non-empty {...} blocks:
      if (!node.is('atrulers') && !node.is('block') || !node.length)
        return;

      node.eachFor('space', function(whitespaceNode) {
        var spaces = whitespaceNode.content;
        var lastIndex = spaces.lastIndexOf('\n');

        // Do not continue if there is no line break:
        if (lastIndex < 0) return;

        // Number of spaces from beginning of line:
        var spacesLength = spaces.slice(lastIndex + 1).length + 1;
        detected.push(new Array(spacesLength).join(' '));
      });
    });

    return detected;
  }
Detects the value of this option in ast.
@param {Node} ast
@return {Array} List of detected values