wallet/balance is Non-Negative

We're done with our rules. Now we need to add smart functions to our collection and predicate specs from simplest to most complicated.

The simplest smart function we need to add is nonNegative?. This function will make sure that our wallet/balance is non-negative.

The below transaction creates a new function, nonNegative?, and adds it to the wallet/balance predicate. It is missing the smart function code - that part will be up to you!

[
  {
    "_id": ["_predicate/name", "crypto/balance"],
    "spec": ["_fn$nonNegative?"],
    "specDoc": "Balance cannot be negative."
  },
  {
    "_id": "_fn$nonNegative?",
    "name": "nonNegative?",
    "code": "SMART FUNCTION CODE HERE"
  }
]

You will need the following Universal SmartFunctions to write the nonNegative? smart function. Below is a subset of the universal smart functions that are available in Fluree. More information about each function can be found in the Universal SmartFunctions section.

You will need one (or several) of these smart function to write, nonNegative.

Function	Arguments	Example
inc	n optional	(inc)
dec	n optional	(dec)
==	arg1 arg2 ...	(== 1 1 1 1)
+	arg1 arg2 ...	(+ 1 2 3)
-	arg1 arg2 ...	(- 10 9 3)
*	arg1 arg2 ...	(* 90 10 2)
/	arg1 arg2 ...	(/ 36 3 4)
>	arg1 arg2 ...	(> 90 10 2)
<	arg1 arg2 ...	(< 90 10 2)
>=	arg1 arg2 ...	(>= 90 90 10 2)
<	arg1 arg2 ...	(< 2 10 90)
<=	arg1 arg2 ...	(<= 2 10 90 90)
max	arg1 arg2 ...	(max 1 2 3)
min	arg1 arg2 ...	(min 1 2 3)

You will also need one context-specific function(?o), which gets the object (value) of the predicate, wallet/balance that is being edited. For example, if the wallet/balance is 100, (+ 1 (?o)) will return 101.

Write a Smart Function!

Using the smart functions listed above, can you figure out how to create a smart function that checks whether the wallet/balance is greater than zero?