| An interface that defines both lookup protocol and parsing of
symbolic names.
This interface is used by UnicodeSet to resolve $Variable style
references that appear in set patterns. RBBI and Transliteration
both independently implement this interface.
A symbol table maintains two kinds of mappings. The first is
between symbolic names and their values. For example, if the
variable with the name "start" is set to the value "alpha"
(perhaps, though not necessarily, through an expression such as
"$start=alpha"), then the call lookup("start") will return the
char[] array ['a', 'l', 'p', 'h', 'a'].
The second kind of mapping is between character values and
UnicodeMatcher objects. This is used by RuleBasedTransliterator,
which uses characters in the private use area to represent objects
such as UnicodeSets. If U+E015 is mapped to the UnicodeSet [a-z],
then lookupMatcher(0xE015) will return the UnicodeSet [a-z].
Finally, a symbol table defines parsing behavior for symbolic
names. All symbolic names start with the SYMBOL_REF character.
When a parser encounters this character, it calls parseReference()
with the position immediately following the SYMBOL_REF. The symbol
table parses the name, if there is one, and returns it.
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