function findSymbolName (node) {
  const { math } = getMath()
  let n = node

  while (n) {
    if (math.isSymbolNode(n)) {
      return n.name
    }
    n = n.object
  }

  return null
}
Find the symbol name of an AssignmentNode. Recurses into the chain of
objects to the root object.
@param {AssignmentNode} node
@return {string | null} Returns the name when found, else returns null.