Valid Rondo Form?

Rondo Form is a type of musical structure, in which there is a recurring theme/refrain, notated as A. Here are the rules for valid rondo forms:

• Rondo forms always start and end with an A section.
• In between the A sections, there should be contrasting sections notated as B, then C, then D, etc... No letter should be skipped.
• There shouldn't be any repeats in the sequence (such as ABBACCA).

Create a function which validates whether a given string is a valid Rondo Form.

Examples

valid_rondo("ABACADAEAFAGAHAIAJA") ➞ True

valid_rondo("ABA") ➞ True

valid_rondo("ABBACCA") ➞ False

valid_rondo("ACAC") ➞ False

valid_rondo("A") ➞ False

Notes

• Inputs will be given as all uppercase.
• For the purposes of this challenge, accept ABA as valid Rondo forms.