function wfHeadingsAtLeast(ast, file, minLevel) {
  if (!minLevel || minLevel <= 1 || minLevel > 6) {
    return;
  }
  visit(ast, 'heading', function(node) {
    if (generated(node)) {
      return;
    }
    if (node.depth >= minLevel) {
      return false;
    }
    const msg = `First heading level should be at least ${minLevel} ` +
      `was ${node.depth}.`;
    file.message(msg, node);
  });
}
Remark Lint Test - minimum heading level.

@param {Node} ast - Root node.
@param {File} file - Virtual file.
@param {number} minLevel