Remove prefix: Other function instead substring-after

I want to remove the prefix of a variable (if it exists).
Ok, I could use the function

substring-after("$myvar",':')

for this task but I feel that there is a special built-in function for
that purpose. Which function should I use?

Obviously if the variable has no prefix then the variable should be returned:

Sample:

removepref("mypref:somevar") ->  somevar
removepref("somevar")        ->  somevar

Matt
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