Scoping

Symbols that you declare are block-scoped. They only exist inside the block that they are declared in. This code causes an Unidentified Symbol error because the E symbol doesn't exist in the scope of the diagram:

element Sand {
    {
        symbol E Empty
    }
    
    @ => E
    E    @
}

It can be useful to scope different parts of your code so that you can keep track of what different symbols mean:

element Sand {
    {
        symbol W Water
        @ => W
        W    @
    }
    {
        symbol W Wind
        W@_ => ._@
    }
}