<Subsequence> is a new subclass
<sequence>. A subsequence
represents an aliased reference to some part of an existing
sequence. Although they may be created by
make (with required keywords
end:) on one of the instantiable subclasses,
they are more often created by calls of the form
subsequence(sequence, start: 0, end: 3)
end: are optional
keywords which default to the beginning and end,
respectively, of the source sequence. No other new
operations are defined for subsequences, since all necessary
operations are inherited from
Because subsequences are aliased references into other sequences, several properties must be remembered:
The contents of a subsequence are undefined after any destructive operation upon the source sequence.
Destructive operations upon subsequences may be
reflected in the source. The results of
sort! should be expected to affect
the source sequence for vector subsequences.
If the source sequences are instances of
<string>, then the implementation
will use subclasses of
<subsequence> which are also
The implementation tries to insure that subsequences of subsequences can be accessed as efficiently as the original subsequence. (For example, the result of
subsequence(subsequence(source, start: 1), start: 2)
would produce a subsequence identical to the one produced by
subsequence(source, start: 3)
Vector subsequences, like all other vectors, implement constant time element access.
Non-vector subsequences may take non-constant time to create, but will provide constant-time access to the first element. This should produce the best performance provided some element of the subsequence is accessed at least once.