I have a function with the following parameter requirements:

1. There are no required parameters
2. $Credential can optionally be used with any other parameters, or no other parameters
3. $Title, $Username, and $SearchString cannot be used together
4. $DefaultObject and $AsPlainText can only be used with $Title or $Username
5. $DefaultObject and $AsPlainText cannot be used together

Below is what I have so far:

[CmdletBinding(DefaultParameterSetName = 'All')]
param (
    [PSCredential] $Credential,
    [Parameter(ParameterSetName = 'Title')]
    [string] $Title,
    [Parameter(ParameterSetName = 'Username')]
    [string] $Username,
    [Parameter(ParameterSetName = 'Search')]
    [string] $SearchString,
    [Parameter(ParameterSetName = 'Title')]
    [Parameter(ParameterSetName = 'Username')]
    [Parameter(ParameterSetName = 'DefaultObject')]
    [switch] $DefaultObject,
    [Parameter(ParameterSetName = 'Title')]
    [Parameter(ParameterSetName = 'Username')]
    [Parameter(ParameterSetName = 'PlainText')]
    [switch] $AsPlainText
)

I have accomplished items 1-4 on the list, but I am struggling with item 5 to prevent $DefaultObject and $AsPlainText from being called simultaneously. Does anybody know how to prevent them from being used together? I have already tried creating a new ParameterSet for each of them, but that did not work.