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|ERROR: Undefined procedure: Name/Arity|
There are three common things people use that produce this error while they do not realise they made a mistake.
Sometimes you want to change a predicate at runtime using assert/1 and retract/1, but you call the predicate before the asserting the first clause. Prolog doesn't know about your intend and reports an error. It is good practice to define every predicate you want to manipulate as dynamic:
:- dynamic is_a/2, person/1.
SWI-Prolog (in fact almost any Prolog system) interprets terms typed to the ?- prompt as queries and wants to prove (i.e., run) them. The preferred way is to use an editor to create a file and load this into Prolog by putting it between square brackets (see also LoadProgram).
If you insist on typing your program at the prompt, consult
the sequence below. Instead of the literal end_of_file term, you can
also type the end-of-file character of your system. This is often
Control-D. Note that in `user consult' mode, the prompt is changed
from ?- into |:.
?- [user]. |: likes(mary, john). |: end_of_file. % user compiled 0.00 sec, 344 bytes