Wait for the Shield Bearers

There was an incident involving a certain
student who came before Yehoshua.

Student	Is the evening prayer optional or cumpulsory?
Yehoshua	It is optional.

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Then the student came to R' Gamliel.

Student	Is the evening prayer optional or cumpulsory?
Gamliel	It is compulsory.
Student	But Yehoshua told me it optional!
Gamliel	Wait until the shield-bearers (Torah scholars) enter the study hall.