Commas in multi-component subscripts and superscripts are sometimes omitted; this is also potentially ambiguous notation. These are limited to one typographic line of symbols, which may include subscripts and superscripts. Usage of subscripts and superscripts is also an important convention. The subscripts can be integers or variables. All non-numeric subscripts are stored in alphabetical order following the numbers. A scalar can be thought of as an array element with zero subscripts. In PDEs, it is common to denote partial derivatives using subscripts. Numeric subscripts (including floating-point numbers) are stored from lowest to highest. The subscripts and denote the incident and scattered waves. The subscripts "l", "v" and "i" denote liquid phase, vapor phase and vapor-liquid interface, respectively. Each function has variables with three subscripts as its arguments. Formally, is a "symmetric polynomial" if for any permutation of the subscripts one has . When combined these subscripts are applied recursively. Here, the numerical subscripts indicate which box the material is in. Both "globals" and "locals" can have child nodes (called "subscripts" in MUMPS terminology).