Get-Module Pester, m | Remove-Module
Import-Module Pester -MaximumVersion 4.9.9
New-Module -Name m -ScriptBlock {
    function a { "hello" }
    function b { a }

    Export-ModuleMember -Function b
} | Import-Module


Describe "m" {
    It "can't see 'a'" {
        { a } | Should -Throw
    }

    It "but it can mock 'a'" {
        Mock a -ModuleName m { "fuuuu" }

        b | Should -Be "fuuuu"
    }
}