A simple substitution: abc

Remember that set returns the new value of the variable:
    X: def Y: def

Note curly braces: [set x "String within quotes within braces"]

See how the set is executed: String within braces within quotes
$x is: String within braces within quotes

Note the \ escapes the bracket:
 $b is: [set y {This is a string within braces within quotes}]
$y is: def