An isogram is a word that has no repeating letters, consecutive or non-consecutive. Implement a function that determines whether a string that contains only letters is an isogram. Assume the empty string is an isogram. Ignore letter case.

Example: (Input --> Output)

"Dermatoglyphics" --> true "aba" --> false "moOse" --> false (ignore letter case)

isIsogram "Dermatoglyphics" = true

isIsogram "moose" = false

isIsogram "aba" = false