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);