What is the meaning of Well-formed_formula?

5-letter words containing

• 5-letter words containing a

• 5-letter words containing b

• 5-letter words containing k

• 5-letter words containing i

• 5-letter words containing r

5-letter words starting with

• 5-letter words starting with d

• 5-letter words starting with p

• 5-letter words starting with r

• 5-letter words starting with g

• 5-letter words starting with p

5-letter words in the middle

• 5-letter words in the middle c

• 5-letter words in the middle e

• 5-letter words in the middle m

• 5-letter words in the middle f

• 5-letter words in the middle i

5-letter words ending with

• 5-letter words ending with k

• 5-letter words ending with u

• 5-letter words ending with s

• 5-letter words ending with e

• 5-letter words ending with o

5-letter words excluding

• 5-letter words excluding d

• 5-letter words excluding u

• 5-letter words excluding n

• 5-letter words excluding u

• 5-letter words excluding g