  Creation operators act on symmetric functions to build Schur functions,
Hall--Littlewood polynomials, and related symmetric functions one row at a
time. Haglund, Morse, Zabrocki, and others have studied more general symmetric
functions $H_{\alpha}$, $C_{\alpha}$, and $B_{\alpha}$ obtained by applying any
sequence of creation operators to $1$. We develop new combinatorial models for
the Schur expansions of these and related symmetric functions using objects
called abacus-histories. These formulas arise by chaining together smaller
abacus-histories that encode the effect of an individual creation operator on a
given Schur function. We give a similar treatment for operators such as
multiplication by $h_m$, $h_m^{\perp}$, $\omega$, etc., which serve as building
blocks to construct the creation operators. We use involutions on
abacus-histories to give bijective proofs of properties of the Bernstein
creation operator and Hall-Littlewood polynomials indexed by three-row
partitions.
