The wasBinarySearchTreePredecessor function takes as arguments:

• a binary search tree (BST) as a flattened list as per binary_trees.
• the node (node) to find the predecessor of.

and returns the in-order predecessor of the specified node in the binary search tree.

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//    Copyright (C) 2013 Wizardry and Steamworks - License: GNU GPLv3    //
///////////////////////////////////////////////////////////////////////////
string wasBinarySearchTreePredecessor(list BST, string node) {
    string left = wasBinaryTreeLeft(BST, node);
    if(left != "") return wasBinarySearchTreeMinimum(BST, left);
    string parent = wasBinaryTreeParent(BST, node);
    while(parent != "" && node == wasBinaryTreeLeft(BST, parent)) {
        node = wasBinaryTreeParent(BST, node);
        parent = wasBinaryTreeParent(BST, parent);
    }
    return parent;
}