private void RemoveSimpleKey()
{
var key = simpleKeys.Peek();
if (key.IsPossible && key.IsRequired)
{
// If the key is required, it is an error.
throw new SyntaxErrorException(key.Mark, key.Mark, "While scanning a simple key, could not find expected ':'.");
}
// Remove the key from the stack.
key.IsPossible = false;
}