In lambdatalk every expression is a word or an S-expression, so:

{b hello world}     // means "boldify the words hello and world"
-> < b>hello world< /b>     // HTML expression

{+ hello world}     // means "add the words hello and world"
-> NaN              // can't do the job and returns Not a Number

{+ 123 1}           // means "add the words 123 and 1"
-> 124              // can do the job and returns the result as a word