How to use the mk.mergeConditions function in mk

def callbackVar(cnd, expr, use_options, target, add_dict):
        if expr in mk.cond_vars:
            cond = mk.cond_vars[expr]
            var = mk.CondVar(makeUniqueCondVarName('%s_%s' % (cond.name, desc)))
            mk.addCondVar(var, hints)
            for v in cond.values:
                cond2 = mk.mergeConditions(cnd, v.cond)
                if '$' in v.value:
                    var.add(v.cond,
                            substitute2(v.value, callback, desc, cond2, hints))
                else:
                    if len(v.value) == 0 or v.value.isspace():
                        var.add(v.cond, v.value)
                    else:
                        var.add(v.cond, callback(cond2, v.value))
            return '$(%s)' % var.name

        if expr in mk.options and mk.options[expr].values != None:
            opt = mk.options[expr]
            var = mk.CondVar(makeUniqueCondVarName('%s_%s' % (opt.name, desc)))
            mk.addCondVar(var, hints)
            for v in opt.values:
                cond = mk.makeCondition("%s=='%s'" % (opt.name, v))

var.add(v.cond,
                            substitute2(v.value, callback, desc, cond2, hints))
                else:
                    if len(v.value) == 0 or v.value.isspace():
                        var.add(v.cond, v.value)
                    else:
                        var.add(v.cond, callback(cond2, v.value))
            return '$(%s)' % var.name

        if expr in mk.options and mk.options[expr].values != None:
            opt = mk.options[expr]
            var = mk.CondVar(makeUniqueCondVarName('%s_%s' % (opt.name, desc)))
            mk.addCondVar(var, hints)
            for v in opt.values:
                cond = mk.makeCondition("%s=='%s'" % (opt.name, v))
                cond2 = mk.mergeConditions(cnd, cond)
                if '$' in v:
                    var.add(cond, substitute2(v, callback, desc, cond2, hints))
                else:
                    if len(v) == 0 or v.isspace(): var.add(cond, v)
                    else: var.add(cond, callback(cond2, v))
            return '$(%s)' % var.name

        if expr in __substituteCallbacks:
            for func in __substituteCallbacks[expr]:
                rval = func(expr, callback, caller)
                if rval != None:
                    return rval
        raise errors.Error("'%s' can't be used in this context, "%expr +
                 "not a conditional variable or option with listed values")