<language, data> A representation of data (typically either a message
passing over a communications link or a program being compiled) which is
independent of machine-oriented structures and encodings and also of the
physical representation of the data (called "concrete syntax" in the case of
compilation or "transfer syntax" in communications).
A compiler's internal representation of a program will typically be specified by
an abstract syntax in terms of categories such as "statement", "expression" and
"identifier". This is independent of the source syntax (concrete syntax) of the
language being compiled (though it will often be very similar). A parse tree is
similar to an abstract syntax tree but it will typically also contain features
such as parentheses which are syntactically significant but which are implicit
in the structure of the abstract syntax tree.
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