[FEEDBACK] Conformance for currency and unit should be more strict

[sffc]

Following up to my #977 audit:

The value of the operand's currency MUST be either a string containing a well-formed Unicode Currency Identifier or an implementation-defined currency type. Although currency codes are expected to be uppercase, implementations SHOULD treat them in a case-insensitive manner.

Why is this SHOULD and not MUST? Pick a behavior but don't let this be implementation-defined.

The value of the operand's unit SHOULD be either a string containing a valid Unit Identifier or an implementation-defined unit type.

Huh? So the operand is a TR35 Unit ID or an implementation-defined unit type, but hey, it's not necessarily either one due to the SHOULD?

I suggest:

1. The string MUST be a TR35 unit identifier
2. An implementation MAY allow other syntaxes (although I prefer to just be strict and require TR35 unit IDs)

[aphillips]

On the :unit issue, we didn't follow our own template here. It should actually say something like:

The value of the operand's unit is either an implementation-defined unit type or a literal consisting of a valid Unit Identifier.

[macchiati]

Agreed on the :unit issue.

On currency, a well-formed Unicode Currency ID is [A-Za-z]{3}, so case insensitive.

We can just drop the line "Although currency codes are expected to be uppercase, implementations SHOULD treat them in a case-insensitive manner."