internal static dynamic ResolveSymbol(Symbol sym, object scope)
 {
     return ((SilverScope) scope)[sym];
 }
 internal VariableExpression(Symbol sym)
 {
     HasSym = true;
     Sym = sym;
 }
 public static VariableExpression Variable(Symbol sym)
 {
     return new VariableExpression(sym);
 }