Learned from the crosswalk by
data-raw/partvii-validation/10-pattern-rules.R. A head is a crosswalk
variant at the end (position "suffix") or start ("prefix") of a title.
Each rule maps titles with that longest head to title.standard: either
the standard most known titles with that head map to (rule "majority")
or the head's own standard when those titles share its role level
("level"). support is the number of known titles behind the rule and
precision the share of them it gets right. Used by
standardize_titles() through match_title_patterns(); load it with
get_title_patterns().