The precedence of any new notation or operator is determined by examining the components from which it is constructed. For instance, is grouped according to the precedence of , the operator is grouped according to the precedence of , and the mapping is grouped according to the precedence of . Generally the grouping behavior of positioning boxes is determined by the "base element". For instance, the expression SubscriptBox[symb,sub] is grouped according to symb. But for some other structural boxes the grouping behavior of surrounding elements is not affected by the behavior of the contents of the box. The precedence of compound objects is determined according to the following table:
type of box
|AdjustmentBox, ErrorBox, StyleBox, SubscriptBox, SuperscriptBox, SubsuperscriptBox, UnderscriptBox, OverscriptBox, UnderoverscriptBox, TagBox||these box structures do not isolate their contents from the outside; the contents of these boxes can affect the grouping of surrounding elements|
|ButtonBox, FormBox, FractionBox, FrameBox, GridBox, InterpretationBox, RowBox, RadicalBox, SqrtBox||these box structures isolate their contents from the outside; the contents of these boxes do not affect the grouping of surrounding elements|
The design decision of making the precedence of new compound operators correspond to their constituents makes intuitive sense and generally leads to notations that are consistent. For instance, consider a possible notation for addition and multiplication over a ring .
The ring multiplication operator has a higher precedence than the ring addition operator because has a higher precedence than . Moreover, the notation is automatically set up to parenthesize the expression appropriately to maintain the correct structure.
It is important to point out that bracketing expressions by using , , , , , or other braces in a notation not only changes the grouping of the notation but requires the brackets to be literally present in the input expression.
You can isolate the elements around an operator or expression by surrounding the operator or expression with a box structure that isolates its contents (compare to the table above). The box structure typically used to do this encapsulation/isolation is the TagBox. Furthermore, it is possible to change the precedence of an operator through the use of the TagBox option SyntaxForm. More will be said on this later in "Complex Patterns and Advanced Features".
The embedded TagBox in Domain∂ is necessary for several reasons: to contain the StyleBox that specifies ZeroWidthTimes as well the styling, to suppress the natural prefix operator grouping of , and to allow the parsing and formatting rules to act on an encapsulated unit.