/// <summary>
/// Create a new child Symbol context and enters it
/// </summary>
public void CreateParentContext(string id = null, CtxKey key = default(CtxKey), bool shadowing = true)
{
var ctx = new SymbolContext <SymbolT, CtxKey>(id, key, shadowing);
current.AddParent(ctx);
numContexts++;
EnterContext(ctx);
}