Sharepoint break role inherentence and add custom permission in sharepoint library/List through Powershell

$snapin = Get-PSSnapin | Where-Object {$_.Name -eq 'Microsoft.SharePoint.Powershell'}
if ($snapin -eq $null) {
Write-Host "Loading SharePoint Powershell Snapin"
Add-PSSnapin "Microsoft.SharePoint.Powershell"
}



function AddGroupsToList($groupName,$permissionLevel)
{
    $Group = $web.SiteGroups[$groupName]
    $GroupAssignment = new-object Microsoft.SharePoint.SPRoleAssignment($Group)
    #Get the permission levels to apply to the new groups
    $RoleDefinition = $web.Site.RootWeb.RoleDefinitions[$permissionLevel]
    #Assign the groups the appropriate permission level
    $GroupAssignment.RoleDefinitionBindings.Add($RoleDefinition)
    #Add the group to the site with the permission level
    $list.RoleAssignments.Add($GroupAssignment)    
    $list.Update()
}


$site = Get-SPSite "http://webapplication/sites/RECMRepository/"
$web = $site.RootWeb



#Break Roleinherentence for "Public Documents" list
Write-Host  -foreground "green" "Breaking Role inherentence for the list - Public Documents"
$list = $web.Lists["Public Documents"]
$list.BreakRoleInheritance($false)
#Add Groups to the list with permission levels assigned
AddGroupsToList "Group1" "Full Control"
AddGroupsToList "Group2" "Contribute"
AddGroupsToList "Group3" "Read"


Note:Breaking role inherentence of either site or library or list with Parameter "False" will give you with clean slate no groups from top level will be copied hence I am adding the groups which I need later,

But if the parameter is "True" it will copy the groups from top level,in the case you may need to delete the unwanted groups from the list