Not a cry of frustration at yet another error message,
but rather, an abbreviation for argument, and also the name
of a built-in metalogical predicate, that extracts arguments
from a structure. Example:
?- arg(2, buys(john, beer), Arg).
Arg = beer
It is also possible to put arguments into terms using
arg, should this prove useful:
?- X = likes(mary, Y), arg(2, X, pizza).
X = likes(mary, pizza)
Y = pizza