/// <summary>
/// Insert a copy of the given argument into this expression
/// </summary>
/// <param name="index"></param>
/// <param name="expr"></param>
/// <returns>The argument that was added to this expression</returns>
public SteExpression InsertCopy(int index, SteExpression expr)
{
if (ReferenceEquals(_argsList, null))
{
throw new InvalidOperationException("Cannot add an argument to an atomic expression");
}
var arg = expr.CreateCopy();
_argsList.Insert(index, arg);
return(arg);
}