parse(file) {
    this.freeze()
    const realFile = vfile(file)
    const parser = this.parser || this.Parser
    assertParser('parse', parser)
    return parser(String(realFile), realFile)
  }
Parse text to a syntax tree.

> **Note**: `parse` freezes the processor if not already *frozen*.

> **Note**: `parse` performs the parse phase, not the run phase or other
> phases.

@param {Compatible | undefined} [file]
  file to parse (optional); typically `string` or `VFile`; any value
  accepted as `x` in `new VFile(x)`.
@returns {ParseTree extends undefined ? Node : ParseTree}
  Syntax tree representing `file`.