This line in MUMPS is not only valid, it also actually does something potentially useful:

    s:foo'="" foo(foo)=foo

It's actually a bit tricky to explain what it does. Everything in MUMPS is effectively a tree. Each tree has a value in the "root node" and you can set it like this:

    set foo="hello"

You can also put data deeper into the tree:

    set foo("fizz")="buzz"

So if something is passed to you and you want to know if you can treat it as a string, you test to see if it's a null string:

    if foo'="" do [something]

('= means !=)

Two more features: 1) Almost every directive can be reduced to one letter. 2) If I want to quit based on a condition, I can do either if condition quit or quit:condition

So the line above is more legibly written as

    if foo'=""  set foo(foo)=foo

So if foo="bar", it's equivalent to:

    set foo("bar")="bar"