question about colon ":" usage in Bash

["vichy" <vichy.kuo@xxxxxxxxx>]

Dear all:
According to the reference document, colon, ":", in bash is used as null command and I can realize this by following example:
    if condition
    then : # Do nothing and branch ahead
    else
    take−some−action
    fi
But how about the following ex1 and ex2 mean?
Ex1: 
: ${username=`whoami`}
${username=`whoami`}

Ex2:
: $((n = $n + 1))
(( n = n + 1 ))

Appreciate your help,
vichy


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