+ * 'hide-if' -- expression given as an array stating when the field
+ * should be hidden. The first array value has to be the
+ * expression's logic operator. Supported expressions:
+ * 'NOT'
+ * [ 'NOT', array $expression ]
+ * To hide a field if a given expression is not true.
+ * '==='
+ * [ '===', string $fieldName, string $value ]
+ * To hide a field if another field identified by
+ * $field has the value $value.
+ * '!=='
+ * [ '!==', string $fieldName, string $value ]
+ * Same as [ 'NOT', [ '===', $fieldName, $value ]
+ * 'OR', 'AND', 'NOR', 'NAND'
+ * [ 'XXX', array $expression1, ..., array $expressionN ]
+ * To hide a field if one or more (OR), all (AND),
+ * neither (NOR) or not all (NAND) given expressions
+ * are evaluated as true.
+ * The expressions will be given to a JavaScript frontend
+ * module which will continually update the field's
+ * visibility.