function gedcom(Prism) {
  Prism.languages.gedcom = {
    'line-value': {
      // Preceded by level, optional pointer, and tag
      pattern:
        /(^[\t ]*\d+ +(?:@\w[\w!"$%&'()*+,\-./:;<=>?[\\\]^`{|}~\x80-\xfe #]*@ +)?\w+ ).+/m,
      lookbehind: true,
      inside: {
        pointer: {
          pattern: /^@\w[\w!"$%&'()*+,\-./:;<=>?[\\\]^`{|}~\x80-\xfe #]*@$/,
          alias: 'variable'
        }
      }
    },
    tag: {
      // Preceded by level and optional pointer
      pattern:
        /(^[\t ]*\d+ +(?:@\w[\w!"$%&'()*+,\-./:;<=>?[\\\]^`{|}~\x80-\xfe #]*@ +)?)\w+/m,
      lookbehind: true,
      alias: 'string'
    },
    level: {
      pattern: /(^[\t ]*)\d+/m,
      lookbehind: true,
      alias: 'number'
    },
    pointer: {
      pattern: /@\w[\w!"$%&'()*+,\-./:;<=>?[\\\]^`{|}~\x80-\xfe #]*@/,
      alias: 'variable'
    }
  }
}
Adds an alias to the given token.

@param {Token} token
@param {string} alias
@returns {void}