Translation between different logics [Exam 2018, February]

Question

Problem 9 (φ4): Translation between different logics [ Exam: 2018, February 14th]

https://es.cs.uni-kl.de/teaching/vrs/exams/2018.02.14.vrs/2018.02.14.vrs.solutions.pdf

φ4 = EF([¬a U Gb]∧FG(b))

Solution: 
EF([¬a U Gb] ∧ FG(b))
=EF[¬a SUGb]  //Since [a U Gb]∧ FG(b) Is equivalent to [¬a SU Gb]. 

= EFE[¬a SU Gb]
= EFE[¬a SU EGb]

EFE[¬a SU EGb]

My doubt is : as per the comment in the solutions: [a U Gb]∧ FG(b) is equivalent to [¬a SU Gb] but  [¬a U Gb] ≠ [a U Gb] (As the former says ¬a holds until always b holds OR ¬a holds forever and the latter says: a holds until always b holds OR a holds forever).

 Then how did the substitution happen?

Answer 1

In the comment, a negation symbol is missing. It should be [¬a U Gb]∧ FG(b) Is equivalent to [¬a SU Gb]