What is this syntax `<-` in `lodash-match-pattern`

I am looking for a library to do json match https://github.com/mjhm/lodash-match-pattern#73-map-values-transform-example.

What I don't understand is the syntax it uses like

var extraFancyResult = matchPattern(realTestData, `{
  name: /^[A-Z]\w+$/,
  email: _.isEmail,
  age: _.isBetween|20|30,
  friends: {
    <-.filterPattern|'{age: _.isBetween|20|30 ...}': _.isSize|2,
    <=.get|name: [
      'Breeze',
      ...
    ]
  }
}`);

The confusion part for me is <-. <=. used in matchPattern:

<-.filterPattern|'{age: _.isBetween|20|30 ...}': _.isSize|2,
    <=.get|name: [
      'Breeze',
      ...
    ]

what kind of syntax it uses?