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Prolog (SWI), 81 70 bytes \X,[I]-->[H],{copy_term(X,Z),\(Z,H,I);nth1(I,X,H)},(\X;[]). a--> \_,!. Try it online! Instead of using predicates, I use definite clause grammar (DCG) notation. DCG notation …

Prolog (SWI), 75 66 bytes B+A,[C]-->[A],{C is A+B},B+1+A;[X],{C=X},1+X. _+_-->[]. a-->0+x,!. Try it online! This program can be run by calling the grammar a//0 to parse the input list. The remainder …

Prolog (SWI), 111 bytes A*B*C:-append(A,B,C). s(A,B,C):-A*[B]*C. N+B:-N=[A|X],X+R,Z*Q*R,maplist(s,[[]|Z],A,Y),Y*Q*B,!;maplist(=([]),B). Try it online! Explanation Let's first look a version of the co …

Prolog (SWI), 109 bytes _+[]+[]. N+[A|B]+[A|C]:-!,N+B+C. \N+B+C:-B=[_|X],N+X+C;C=[_|Y],N+B+Y;B=[_|X],C=[_|Y],N+X+Y. B*C:- \ \ \a+B+C. Try it online! This is a golf of Wheat Wizard's answer. The key m …

Avoid using triggers to sequence messages One of the common uses of trigger objects is to make the order of execution clear when a single object is sending messages to multiple places. In those cases …

Prolog (SWI), 74 bytes \D-->[X,Y],{D is 2*X+Y};[D];a,\D. a,[A],[B]-->[X,Y,Z],{A is X+Y,B is X+Z}. Try it online! Takes input as a list of integers 1 and 0 with the most significant bit first.

Prolog (SWI), 649 188 bytes Unflipped, Unrotated :-% l :%r %-: write(%e%-(i %(etirw left).%f.weg%.)right % t)rth. % % )tiit) % % .htr)t % right).%gew.f%.)left wr …

Prolog (SWI), 109 bytes []+[]+[]. I+[B|X]+[C|O]:-(I=[B|J],B=C;I=J,C=45),J+X+O. I-O:-I+`Ah, I see you're a man of culture as well.`+O. Try it online! Case-sensitive and whitespace sensitive. The -/ …

Prolog (SWI), 64 bytes A+B:-findall(X,(append(X,Y,A),append(Y,X,A)),[_|Z]),length(Z,B). Try it online! Defines a predicate +/2 that takes a string (in the form of a list of character codes) as its …

Prolog (SWI), 29 bytes a-->[X],+X. +X-->[];a,[X],+X. Try it online! This program defines a DCG rule a//0 that matches any string (list of characters) that is a mountainous string. Explanation Fo …

Prolog (SWI), 574 bytes Z*A:-findall(X^Y-D,(nth0(Y,Z,O),nth0(X,O,C),plus(32,C,D)),L),list_to_assoc(L,A). N^C^G^[P,I|R]^F^X^Y^D:-map_assoc(\=(74),G);N<1e3,get_assoc(X^Y,G,J),J>67,put_assoc(X^Y,G,78,H) …

Prolog (SWI), 132 bytes +[]-->[]. +[47,124,92|T]-->"/|\\",+T. +[47,47|T]-->"/|",+T. +[92,92|T]-->"|\\",+T. +[X|T]-->[X],+T. X+Y:- +(N,X,[]),!,(X=N,Y=N;N+Y). Try it online! This program defines a p …

Use Operators for Predicate Names It is possible to give predicates operators as names so long as the operator is one of the predefined operators (listed here) and is not already defined as a predicat …

Prolog (SWI), 27 bytes +N:-between(0,N,I),N=:=I*I. Try it online! Explanation Searches through all numbers greater or equal to 0 and less than or equal to N and tests whether that number squared …

Prolog (SWI), 42 40 39 bytes +X:-X>1,2+X. Y+X:-X=<Y;0<X mod Y,1+Y+X. Try it online! Tests primality by checking whether any of the numbers less than the number and greater than one divide the number. …