prolog - Undefined procedure when procedure is defined -

i trying define simple binary search tree. stored in lists so: [key, left tree, right tree]. believe have done when try use bstadd on existing tree following error.

?- bstadd(19,[],t1), bstadd(9, t1, t2). error: bstadd/3: undefined procedure: right/3    exception: (8) right(9, [[], []], _g3233) ? 

i have defined right 3 arugments on line 8. follows code:

% bstadd(key, tree, newtree) % add element key tree tree , return  % new tree newtree. element in left subtree l must less key ,  % elements in right subtree r must greater key.  means duplicates  % not allowed in binary search tree. don’t put print statements in  % predicate.  right(key, [treekey|treetail], [treekey|newtree]) :- grabtail(key, treetail, newtree]). grabtail(key, [treekey|_], [treekey|newtree]) :- bstadd(key, treekey, newtree). bstadd(key, [], [key,[],[]]). bstadd(key, [treekey|treetail], [treekey|newtree]) :- key > treekey, grabtail(key, treetail, newtree). bstadd(key, [treekey|treetail], [treekey|newtree]) :- key < treekey, right(key, treetail, newtree).   % inorder(tree)  % given binary search tree tree perform inorder traversal of  % tree printing (use print(x) ) value of each vertex inorder. inorder([treehead|treetail]) :- inright(treetail), print(treehead), intail(treetail). inright([_|treetail]) :- intail(treetail). intail([treehead|_]) :- inorder(treehead). 

any , insight appreciated.

always systems says while loading file. sure there kind of syntax error. in case, right/3 contains syntax error @ end. there unmatched closing ].

right(key, [treekey|treetail], [treekey|newtree]) :- grabtail(key, treetail, newtree]).                                                                                    ^^^