trans - arbitrary transition generator

trans output[b] t1[dvp] t2[dpv] t3[dpv] [t[vpn] v[vpn] a[vpn]]* t[vpn] v[vpn] ;

trans is the unit generator equivalent of gen4, which is a general
transition generator.  The three temporary variables, t1, t2, and t3,
are normally d's.  The rest of the line gives a description of a
transition path exactly like a gen4 definition, except that the
transition path occurs over the duration of a note.  Any number of
time-value (t-v) pairs may be given, and the transition parameters
(a's) work as they do in gen4: 0 yields a straightline transition,
negative numbers yield exponential transitions, and positive numbers
yield inverse exponential transitions.  The power of this generator
lies in the fact that the transition points and parameters may be
connected to p-fields, allowing such things as easy glissando
specification, loudness contouring, etc.  For example, the following
score plays a 1-second glissando from 440Hz to 880Hz, followed by a
2-second gliss to 100Hz:
    ins 0 gliss;
	trans b1 d d d 0,p6 0 1/3,p7 0 1,p8;
	osc   b1 p5 b1 f1 d;
	out   b1;
    end;
    gen 0 gen5 f1 1 1 0;
    note 0 gliss 3 -6dB 440Hz 880Hz 100Hz;
    ter;
