TASK DEFINITION: In this task, you need to replace a letter in the sentence with another given letter.
PROBLEM: Sentence: 'a train traveling through a dark tunnel next to green brush.'. Replace the letter 'r' with 'e' in the sentence.

SOLUTION: a teain teaveling theough a daek tunnel next to geeen beush.

PROBLEM: Sentence: 'the man is skiing fast in the snow'. Replace the letter 'o' with 'x' in the sentence.

SOLUTION: the man is skiing fast in the snxw

PROBLEM: Sentence: 'a section of a yard being dug up next to a building'. Replace the letter 'y' with 'l' in the sentence.

SOLUTION:
a section of a lard being dug up next to a building