+ <para>
+ Invokes <command>letin H ≝ (t ? ... ?)</command>
+ with enough <command>?</command>'s to reach the
+ <command>d</command>-th independent premise of
+ <command>t</command>
+ (<command>d</command> is maximum if unspecified).
+ Then istantiates (by <command>apply</command>) with
+ t<subscript>1</subscript>, ..., t<subscript>n</subscript>
+ the <command>?</command>'s corresponding to the first
+ <command>n</command> independent premises of
+ <command>t</command>.
+ Usually the other <command>?</command>'s preceding the
+ <command>n</command>-th independent premise of
+ <command>t</command> are istantiated as a consequence.
+ If the <command>linear</command> flag is specified and if
+ <command>t, t<subscript>1</subscript>, ..., t<subscript>n</subscript></command>
+ are (applications of) premises in the current context, they are
+ <command>clear</command>ed.
+ </para>