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.