function Get-SetDifference {
<#
.SYNOPSIS
	Returns List $a minus the entries of List $b
#>
	[CmdletBinding()]
	Param (
		[Parameter(Mandatory=$true)]
		[AllowNull()]
		[object]$a,
		[Parameter(Mandatory=$true)]
		[AllowNull()]
		[object]$b
	)

	if(!$a)
	{
		return $null
	}
	if(!$b)
	{
		return $a
	}

	$results = @();
	$map = @{};

	foreach($key in $b)	{
		$map.Add($key, '1')
	}

	foreach($key in $a)	{
		if(!($map.ContainsKey($key))) {
			$results += $key
		}
	}

	return $results
}