The interleave problem

When you begin using Relax NG to describe mixed content, you can come up with this error in your grammar:

both operands of "interleave" contain "text"

Here is what causes this error. Consider a simple weather report that consists of mixed text, with one or more places, low temperatures, and high temperatures.

<report>
Here's your weather report.  The temperatures
for <place>San Jose</place> will be from <low>75</low> to
<high>80</high>. <place>San Francisco</place> will have
temperatures ranging from <low>55</low> to <high>60</high>.
</report>

Here is a correct Relax NG grammar in compact syntax:

grammar 
{
    start = element report 
    {
        text &
        (
            element place { text } &
            element low { xsd:integer } &
            element high { xsd:integer }
        )+
    }
}

However, you might think: “There’s text between the <place>, <low> and <high>. So shouldn’t I write my grammar this way:” (lines are numbered for reference; line 5 is the one you added):

 1 grammar {
 2  start=element report
 3  {
 4      text &
 5      (
 6          text &
 7          element place { text } &
 8          element low { xsd:integer } &
 9          element high { xsd:integer }
10      )+
11  }
12 }
 

No, you can’t. That will generate the error. You can have only one {text} defined in an element. You already have one on line 4; adding the one on line 6 causes the problem. The {text} on line 7 is just fine, because it is enclosed in a different element

By the way: the line number in the error message doesn’t give you a clue as to where the error really occurs; you will have to track it down yourself.