code: func (ctx NSContext) LookupPrefix(prefix string) (string, error) {
  	if namespace, ok := ctx.prefixes[prefix]; ok {
  		return namespace, nil
  	}
  
  	return "", ErrUndeclaredNSPrefix{
  		Prefix: prefix,
  	}
  }
docs: // LookupPrefix attempts to find a declared namespace for the specified prefix. If the prefix
  // is an empty string this will be the default namespace for this context. If the prefix is
  // undeclared in this context an ErrUndeclaredNSPrefix will be returned.