MathematicaScalar ISymbolicVector.this[int id] => TermsTree.GetLeafValue((ulong)id)?.ToMathematicaScalar() ?? SymbolicUtils.Constants.Zero;
public Expr this[int id] => TermsTree.GetLeafValue((ulong)id) ?? Expr.INT_ZERO;
public double this[int id] => TermsTree.GetLeafValue((ulong)id);