Правы MSM и M3-Lite, Cache - нет, согласно
1995 Version of ANSI (Equivalent to Current ISO Version) of StandardFor each setleft that is a setpiece, of the form $Piece(glvn,d,m,n), the value of expr replaces the mth through the nth pieces of the current value of the glvn, where the value of d is the piece delimiter. Note that both m and n are optional. If neither is present, then m = n = 1; if only m is present, then n = m. If glvn has no current value, the empty string is used as its current value. Note that the current value of glvn is obtained just prior to replacing it. That is, the other arguments of setpiece are evaluated in left-to-right order, and the expr to the right of the = is evaluated prior to obtaining the value of glvn.
Let s be the current value of glvn, k be the number of occurrences of d in s, that is, k = max(0,$Length(s,d) - 1), and t be the value of expr. The following cases are defined, using the concatenation operator _ of 7.2.1.1:
m > n or n < 1.
The glvn is not changed and does not change the naked indicator.
n '< m–1 > k.
The value in glvn is replaced by s_F(m–1-k)_t, where F(x) denotes a string of x occurrences of d, when x > 0; otherwise, F(x) = "". In either case, glvn affects the naked indicator.
m–1 '> k < n.
The value in glvn is replaced by $P(s,d,1,m–1)_F(min(m–1,1))_t.
Otherwise,
The value in glvn is replaced by $P(s,d,1,m–1)_F(min(m–1,1))_t_d_$P(s,d,n+1,k+1). Цитирую по The Annotated MUMPS Standards. Ed de Moel.

Number of occurrences, по определению: Стандарт... k be the number of occurrences of d in s, that is, k = max(0,$Length(s,d) - 1)$Length(s,"") = 0 => k = max(0, -1) => k = 0. При m>1 имеем m-1 > k, т.е. попадаем в условия варианта: Стандартn '< m–1 > k.
The value in glvn is replaced by s_F(m–1-k)_t, where F(x) denotes a string of x occurrences of d, when x > 0; otherwise, F(x) = "".Имеем F(m-1-k)="", поэтому результат равен s_t.