<aura:application extends="force:slds">
    <aura:attribute name="value" type="Integer" default="{!1+1}" />

    <c:test value="2" />           //changeable
    <c:test value="{!2}" />        //changeable
    <c:test value="{#2}" />        //changeable
    <c:test value="{!1+1}" />      //immutable
    <c:test value="{#1+1}" />      //immutable
    <c:test value="{!v.value}" />  //immutable (same here, should be bound, but not final)
    <c:test value="{#v.value}" />  //changeable
</aura:application>

<aura:component >
    <aura:attribute name="value" type="Integer" />

    <lightning:button label="Change Valuze to 10" onclick="{! c.changeValue }" />
    {!v.value}
</aura:component>

({
    changeValue: function(cmp, evt, helper) {
        cmp.set("v.value", 10);
    },
})