/**
     * Nested Keys File Loading.
     *
     * Given a keys file url, load the file and then recursively
     * composite into it any keys files it points to (somewhat like macro expansion).
     * @return {String} The result of the compositing operation with all reference layers removed.
     * @name jibo.rendering.animation.KeysLoader#innerNestedLoad
     * @param {String} url Path nanme of the keys file to be loaded
     * @param {Callback} complete Callback to call when the load is completed.
     * @param {String} [parentId] The layer ID of the referencing frame.
     * @param {String} [refBase] The directory that the asset references are with respect to.
     * @private
     */

    /**
     * Convert keyframes referencing texture, pixi and audio information into assetpack style references if necessary.
     *
     * The motivation:
     * Consider a reference layer in a skill's animation that references an animDB keys file.
     * Compositing will pull in the layers from the animDB keys file, but any asset references (textures, audio, pixi)
     * in those layers will be relative to the animDB root. If the paths to those assets are not converted to
     * asset-pack style references (while we know their provenance) we will not be able to distinguish later on
     * whether those references are relative to the animDB or to the local skill.
     *
     * @param {Keyframes} frames the keyframes object whose asset references will be modified.
     * @param {string} assetPack the assetPack that the containing file is in.
     */

    /**
     * Convert a non-asset pack reference into an asset-pack one.
     * @param assetPack
     * @param filepath
     * @returns {string}
     */

    /**
     * Find all references in a keys file, returning them in reverse-order.
     * @name jibo.rendering.animation.KeysLoader#getKeyFileReferences
     * @param {Keyframes} frames A keyframes structure containing reference layers.
     * @param {String} refBase The directory containing the file from whence the given keyframes were loaded.
     * @returns {Array} An array of objects describing references to other keys files,
     * each object has these properties:
     * `url`: the url of the file being referenced,
     * `parentId`: the layerID of the parent,
     * `holdFinalPose`: whether the final frame of the referenced layer is held until the next reference (or end),
     * `containerDuration`: the duration of the file that contains this reference.
     * `layerNumber`: the ordinal layer number of the reference,
     * `atEnd`: a boolean that is true if this is the last reference in the layer.
     * `nextRefStart`: where the next reference frame is, or Infinity if there are no further references.
     * `timeOffset`: the timepoint at which it is to be inserted in the referencing layer.
     * @private
     */

    /**
     * Place num in a leading-zero-padded field of the given size
     * @param {number} num Number to pad
     * @param {number} size Size of final field
     * @returns {string}
     */

    /**
     * Return a full URL for the referenced keys file based on
     * the reference mode (by name or by file).
     * @param {any} keyframe the keyframe containing the reference.
     * @param {string} refBase directory of the file in which the reference is occurring.
     * @returns {object} object.url containing the fully resolved path to referenced file and
     * object.refBase containing the parent directory of the referencer.
     * @method jibo.rendering.animation.KeysLoader#getReferencedUrl
     * @private
     */

    /**
     * Find any "HOLD_SAFE" events in referenced layers and remove them (by renaming them).
     * @name jibo.rendering.animation.KeysLoader#removeHoldSafesInReferences
     * @param {Keyframes} frames keyframes to be cleaned.
     * @private
     */

    /**
     * Strip out any reference-type layers from the keyframes.
     * @name jibo.rendering.animation.KeysLoader#removeReferenceLayers
     * @param {Keyframes} keyframes keyframes structure containing reference layers.
     * @private
     */

    /**
     * Shift the layer's validFrom, validUpto and validHoldAfter properties and
     * shift the timepoint of all the keyframes within each of the given layers by the given time offset ("validFrom").
     * The validFrom and validUpto parameters are in "referencing" layer coordinates.
     * @name jibo.rendering.animation.KeysLoader#timeshift
     * @param {Keyframes} frames the layers containing the keyframes to be shifted.
     * @param {number} validFrom the beginning of the valid region.
     * @param {number} validUpto the ending of valid region, or Infinity for unlimited.
     * @param {number} holdAfter the timepoint after which the pose does not change (i.e. it remains fixed at that timepoint).
     * @private
     */

    /**
     * Insert all the layers of toBeInsertedKeyframes into existingKeyframes at the given layer number.
     * Insert them last-first in order to preserve their original ordering at the designated layer.
     * @name jibo.rendering.animation.KeysLoader#composite
     * @param {Number} layerNumber the layer number in which the keyframes are to be inserted.
     * @param {Keyframes} existingKeyframes the existing keyframes into which the layers will be inserted.
     * @param {Keyframes} toBeInsertedKeyframes the source of the keyframes to be inserted.
     * @private
     */

    /**
     * Modifies the layer ids to prefixed by the referencing layer's id (delimited by "-")
     * and suffixed with the layer number (delimited by '.').
     * @name jibo.rendering.animation.KeysLoader#fixLayerIds
     * @param {Keyframes} frames the set of layers to be modified.
     * @param {string} prefix the string with which to prefix each layer id.
     * @private
     */

    /**
     * Was this layer produced from a keyframes reference?
     * @name jibo.rendering.animation.KeysLoader#isReferencedLayer
     * @param {any} layer the layer to be checked
     * @returns {boolean}
     * @private
     */

 /**
    //  * Return true if frame being referenced already exists in hierarchy.
    //  * @param frames
    //  * @param prefix
    //  */