What is the meaning of Formal_system?

5-letter words containing

• 5-letter words containing r

• 5-letter words containing e

• 5-letter words containing a

• 5-letter words containing v

• 5-letter words containing n

5-letter words starting with

• 5-letter words starting with t

• 5-letter words starting with i

• 5-letter words starting with l

• 5-letter words starting with u

• 5-letter words starting with u

5-letter words in the middle

• 5-letter words in the middle i

• 5-letter words in the middle a

• 5-letter words in the middle a

• 5-letter words in the middle c

• 5-letter words in the middle i

5-letter words ending with

• 5-letter words ending with a

• 5-letter words ending with k

• 5-letter words ending with e

• 5-letter words ending with a

• 5-letter words ending with i

5-letter words excluding

• 5-letter words excluding r

• 5-letter words excluding u

• 5-letter words excluding o

• 5-letter words excluding c

• 5-letter words excluding a