/// SYNOPSIS: Builds a 'rule parser' matching a sequence of elements.
/// ARGUMENTS:
///   "#rest sub-rules" - A series of 'rule parser's, all of which must succeed
///                       for the returned parser to succeed.
/// VALUES:
///   rule-parser - A 'rule parser' returning a <sequence>. The sequence will
///                 contain the sub-rules' products.
define function seq (#rest sub-rules) => (rule-parser :: <function>)
...


/// Synopsis: A tree that allows unlimited child nodes, numbered from 0.
/// Make Keywords:
///   fill: - An instance of <object> specifying the initial value for any
///           implicitly-created elements. The default value is #f.
///   root: - An instance of <object> specifying the value of the root node.
///           If omitted, the tree has no root node upon creation.
define class <ordered-tree>
...