Newly introduced, the optional 3rd "operator" argument of SUBSET() can now accept text-specific operators (in addition to the implicit default "=" and other existing numerical operators such as ">="):

• "contains"
  
• "does-not-contain"
  
• "ends-with"
  
• "starts-with"
  

For example, this identifies all people whose surname begins "Mac":

SUBSET([Surname], "Mac", "starts-with")

See also:
http://www.visokio.com/kb/functions-guide#SUBSET
http://www.visokio.com/kb/subset-functions