when-let (bindings &body forms)
Creates new variable bindings, and conditionally executes forms.
bindings must be either single binding of the form:
(variable initial-form)or a list of bindings of the form:
((variable-1 initial-form-1) (variable-2 initial-form-2) ... (variable-n initial-form-n))All initial-forms are executed sequentially in the specified order. Then all the variables are bound to the corresponding values. If all variables were bound to true values, then forms are executed as an implicit progn.