our use of 6 for schwa is nonstandard

[tjmahr]

library("worldbet")
test <- c("bod6jaU","dEgd6ne","vIt6gap","nUb6m6n","motaUk","twEd6mIn")
convert_bet_to_ipa(test)
#[1] "bod"    "dɛgd"   "vɪt"    "nʊb"    "motaʊk" "twɛd" 

Symbol by symbol conversion ends when a match for 6 is not found in the conversion table.

A few different ways to fix this in order of difficultly:

• Add 6 = schwa to table. 6 current handles to the non-English /ɶ/
• Keep the unmatchable WorldBet in the string, but issue a warning.
• Allow users to register their own conversions in the table. Would require a rewrite of the functions, because these conversions are looked up from internal data structures.

I'll do the first for now.