Function: Strexpand
Section: conversions
C-Name: Strexpand
Prototype: s*
Help: Strexpand({x}*): concatenates its (string) argument into a single
 string, performing tilde expansion.
Doc:
 converts its argument list into a
 single character string (type \typ{STR}, the empty string if $x$ is omitted).
 Then perform \idx{environment expansion}, see \secref{se:envir}.
 This feature can be used to read \idx{environment variable} values.
 \bprog
 ? Strexpand("$HOME/doc")
 %1 = "/home/pari/doc"
 @eprog

 The individual arguments are read in string context, see \secref{se:strings}.
 %\syn{NO}
