%% The wrapper predicate. Calls pptree/2 with 0 indentation. pptree(Term) :- pptree(0,Term). %% pptree/2: The first argument says how much a new line has to be %% indented, the second argument is the term to be printed. %% If the term is an atom, print it. pptree(_,A) :- atom(A), write(A). %% If the term is a complex term, print a newline, print as many %% blanks as specified by the first argument, print the functor and an %% opening parenthesis, then call the predicate process_args to print %% the arguments. pptree(Ident,C) :- functor(C,F,A), A > 0, nl, tab(Ident), write(F),write('('), NewIdent is Ident + 3, process_args(1,A,C,NewIdent), write(')'). %% process_args/4: The first two arguments say which argument is %% currently being processed and how many arguments there are all %% together. If these numbers are the same, we are processing the last %% argument. So, just call pptree/1 for this argument. process_args(A,A,C,Ident) :- arg(A,C,AthArg), pptree(Ident,AthArg). %% If the first argument (X) is smaller than the second argument, then %% get the Xth argument of the complex term and print it. Then process %% the remaining args: that is, call process_args with the first %% argument (X) being increased by 1. process_args(X,A,C,Ident) :- X < A, arg(X,C,XthArg), pptree(Ident,XthArg), NewX is X + 1, process_args(NewX,A,C,Ident).