function getOperator (fn) {
  const identifier = 'OperatorNode:' + fn
  for (const group of properties) {
    if (identifier in group) {
      return group[identifier].op
    }
  }
  return null
}
Get the operator associated with a function name.
Returns a string with the operator symbol, or null if the
input is not the name of a function associated with an
operator.

@param {string} Function name
@return {string | null} Associated operator symbol, if any