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# assert(+Clause)

Add specified clause at the end of the dynamic procedure to the database.

Clause
  Atom or compound term

## Description

Adds the specified clause for a dynamic procedure to the database. The clause is added AFTER any existing clauses for the procedure.

The procedure must be declared to be dynamic using the dynamic/1 built-in. If the procedure is undefined an exception is raised. However, the default error handler for this exception simply declares the procedure dynamic and asserts the clause.

The asserted clause is NOT removed from the database on backtracking through the call to assert/1.

assert/1 satisfies the logical update semantics. Asserting a clause to a procedure will not, in any way, affect previous calls to it when backtracking.

### Modes and Determinism

- assert(+) is det

### Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

### Exceptions

(4) instantiation fault
  Clause is a free variable.
(4) instantiation fault
  The head of Clause is a free variable.
(5) type error
  Clause is not a valid Prolog clause.
(63) procedure not dynamic
  The procedure is not dynamic.
(70) accessing an undefined dynamic procedure
  The procedure is undefined. However, the default error handler for this exception simply declares the procedure dynamic and asserts the clause, if the error was caused by an assert.

## Examples

       Success:
        assert(city(munich)).
        assert((likes(X,Y):-man(X),woman(Y)).

       Error:
        assert(X).                        - gives error 4.
        assert("the man").                - gives error 5.
        assert(1).                        - gives error 5.
        assert((my_static(X):-write(X))). - gives error 63.
                                            if my_static/1 is not
                                            dynamic.
       Logical semantics :

       If the following clauses are in the database :
    p :- assert(p), fail.
    p :- fail.

    q :- fail.
    q :- assert(q), fail.

       The queries p.  and q.  will both fail.

## See Also

dynamic / 1, asserta / 1, retract / 1, compile_term / 1