# Compound Syntax

The simplest form of structuring terms is the compound. The
compound consists of a name followed by one or many arguments. The
principal name is called the functor and the number of arguments
is called the arity. To disambiguate from operator expressions a
compound is only recognized when the name directly precedes the
left parenthesis (“(”).

compound --> "{" term(1200) "}"

| atom "(" arguments ")"

| atom "(" ")".

arguments --> term(999) { "," term(999) }.

Examples:

f(a,b) % is a compound

{} % is the empty set, corresponds to '{}'

{a} % is a compound, corresponds to '{}'(a)

-(a,b) % is a compound, corresponds to a - b

- (a, b) % is an expression, corresponds to - ,(a, b)

By means of the braces (“{}”) a form of set notation can be
invoked. The empty set notation is just a short hand for the name
‘{}’. The set notation that surrounds a term is a compound of
arity one with the functor ‘{}’. Since the empty set {} and the
empty list [] do belong to the category of atoms they can be used
as a functor as well.

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