function walkSchema(schema, parent, state, callback) {

    if (typeof state.depth === 'undefined') state = getDefaultState();
    if ((schema === null) || (typeof schema === 'undefined')) return schema;
    if (typeof schema.$ref !== 'undefined') {
        let temp = {$ref:schema.$ref};
        if (state.allowRefSiblings && schema.description) {
            temp.description = schema.description;
        }
        callback(temp,parent,state);
        return temp; // all other properties SHALL be ignored
    }

    if (state.combine) {
        if (schema.allOf && Array.isArray(schema.allOf) && schema.allOf.length === 1) {
            schema = Object.assign({},schema.allOf[0],schema);
            delete schema.allOf;
        }
        if (schema.anyOf && Array.isArray(schema.anyOf) && schema.anyOf.length === 1) {
            schema = Object.assign({},schema.anyOf[0],schema);
            delete schema.anyOf;
        }
        if (schema.oneOf && Array.isArray(schema.oneOf) && schema.oneOf.length === 1) {
            schema = Object.assign({},schema.oneOf[0],schema);
            delete schema.oneOf;
        }
    }

    callback(schema,parent,state);
    if (state.seen.has(schema)) {
        return schema;
    }
    //else
    if ((typeof schema === 'object') && (schema !== null)) state.seen.set(schema,true);
    state.top = false;
    state.depth++;

    if (typeof schema.items !== 'undefined') {
        state.property = 'items';
        walkSchema(schema.items,schema,state,callback);
    }
    if (schema.additionalItems) {
        if (typeof schema.additionalItems === 'object') {
            state.property = 'additionalItems';
            walkSchema(schema.additionalItems,schema,state,callback);
        }
    }
    if (schema.additionalProperties) {
        if (typeof schema.additionalProperties === 'object') {
            state.property = 'additionalProperties';
            walkSchema(schema.additionalProperties,schema,state,callback);
        }
    }
    if (schema.properties) {
        for (let prop in schema.properties) {
            let subSchema = schema.properties[prop];
            state.property = 'properties/'+prop;
            walkSchema(subSchema,schema,state,callback);
        }
    }
    if (schema.patternProperties) {
        for (let prop in schema.patternProperties) {
            let subSchema = schema.patternProperties[prop];
            state.property = 'patternProperties/'+prop;
            walkSchema(subSchema,schema,state,callback);
        }
    }
    if (schema.allOf) {
        for (let index in schema.allOf) {
            let subSchema = schema.allOf[index];
            state.property = 'allOf/'+index;
            walkSchema(subSchema,schema,state,callback);
        }
    }
    if (schema.anyOf) {
        for (let index in schema.anyOf) {
            let subSchema = schema.anyOf[index];
            state.property = 'anyOf/'+index;
            walkSchema(subSchema,schema,state,callback);
        }
    }
    if (schema.oneOf) {
        for (let index in schema.oneOf) {
            let subSchema = schema.oneOf[index];
            state.property = 'oneOf/'+index;
            walkSchema(subSchema,schema,state,callback);
        }
    }
    if (schema.not) {
        state.property = 'not';
        walkSchema(schema.not,schema,state,callback);
    }
    state.depth--;
    return schema;
}
begins the walk of a schema object
@param parent the parent schema, if any. Use empty object if none
@param state the initial starting state of the walker, usually obtained from `getDefaultState`. Use empty object to auto-initialise
@param callback a function taking a schema, parent and state to be called on this and all subschemas
@return the schema object