(decedent owned fractional interests in artwork at time of death that had been placed in GRIT, but had signed an agreement waiving his right to file partition action; decedent survived 10-year term of GRIT with decedent's undivided interest passing equally to decedent's three children with each child receiving 16.67 percent interest in artworks; decedent's spouse died before end of GRIT with her undivided 50 percent interest in the artwork passing to decedent; decedent disclaimed sufficient amount of interest in artwork to optimize use of unified credit so as to pass disclaimed portion to children without estate tax; decedent then entered into agreement with children giving up his right to partition; waiver of such right disregarded for valuation purposes under I.R.C. Sec. 2703(a)(2) - provision states that property value to be determined without regard to any restriction on right to sell or use property; exception from I.R.C. Sec. 2703(a)(2) contained in I.R.C. Sec. 2703(b) held inapplicable; court applied 10 percent discount to pro-rata value of artworks due to uncertainties concerning their value associated with children's intentions concerning the artwork).