Schematron test for internal-destination is wrong

[michael-aka-mmh]

Current error is:

internal-destination="d13e15" token should be 'empty' or 'string'. Enumeration token is 'd13e15'.

It is not "empty or string" but "empty string or idref"

Can we check if referenced id is specified somewhere in the current document?

Spec says:

7.23.8 "internal-destination"
XSL Definition:
Value: empty string |
Initial: empty string
Inherited: no
Percentages: N/A
Media: interactive

Specifies the destination flow object within the formatting object tree. This property allows the destination flow object node to be explicitly specified.

[tgraham-antenna]

In fo.rnc:

internal-destination =
    ## empty string | <idref>
    attribute internal-destination { xsd:IDREF }?

The "empty or string" problem is already in #20.