Function default_priority 

pub fn default_priority(pattern: &Expr) -> f64

Compute the default priority for a pattern AST node. The XSLT spec defines four buckets:

Pattern shape	Priority
prefix:local, @prefix:local	0
NCName:*, @NCName:*	-0.25
*, @*, node(), text(), comment(), pi()	-0.5
anything more specific (paths, predicates, etc.)	0.5