What is the meaning of Well-formed_formula?

5-letter words containing

• 5-letter words containing i

• 5-letter words containing b

• 5-letter words containing n

• 5-letter words containing r

• 5-letter words containing u

5-letter words starting with

• 5-letter words starting with a

• 5-letter words starting with e

• 5-letter words starting with d

• 5-letter words starting with e

• 5-letter words starting with a

5-letter words in the middle

• 5-letter words in the middle n

• 5-letter words in the middle a

• 5-letter words in the middle l

• 5-letter words in the middle c

• 5-letter words in the middle i

5-letter words ending with

• 5-letter words ending with r

• 5-letter words ending with v

• 5-letter words ending with k

• 5-letter words ending with o

• 5-letter words ending with e

5-letter words excluding

• 5-letter words excluding l

• 5-letter words excluding n

• 5-letter words excluding e

• 5-letter words excluding m

• 5-letter words excluding p