Stacks are an important way of supporting nested or recursive function calls. Arrays are rarely nested more than one level deep. JavaScript supports nested labels in most implementations. HTML documents imply a structure of nested HTML elements. All these sequences are properly nested. Nodes may have subnodes, which appear as nested boxes. Gimbals are typically nested one within another to accommodate rotation about multiple axes. One coordinate structure can easily be nested inside another. Multiline comments in Swift can be nested inside other multiline comments. Parentheses may be nested (generally with one set (such as this) inside another set). Scheme programs thus consist of sequences of nested lists. Often, frames of several different sizes are nested inside each other. All the film footage is kept on encrypted drives with multiple levels of nested protection. Sequences may be nested to any level. This means that the call tree is a linear chain of nested calls.