UnDeclareFCTensor[a, b, ...] undeclares
a,b, ... to be tensor heads, i.e.,
DataType[a,b, ..., FCTensor] is set to
False.
Overview, DeclareFCTensor, FCTensor.
ClearAll[myTens]
DeclareFCTensor[myTens]
ExpandScalarProduct[myTens[z, Momentum[a + b], Momentum[c + d]]]\text{myTens}\left(z,\overline{a},\overline{c}\right)+\text{myTens}\left(z,\overline{a},\overline{d}\right)+\text{myTens}\left(z,\overline{b},\overline{c}\right)+\text{myTens}\left(z,\overline{b},\overline{d}\right)
UnDeclareFCTensor[myTens]