code: function instance(schema, defaults, options) {
    const name = 'instance'
    const type = `instance<${schema.name}>`
    const validate = (value = resolveDefaults(defaults)) => {
      return value instanceof schema
        ? [undefined, value]
        : [{ data: value, path: [], value, type }]
    }
  
    return new Kind(name, type, validate)
  }
docs: Instance.
  
  @param {Array} schema
  @param {Any} defaults
  @param {Object} options